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How do I replace only part of an image source?

Is there a way to use jQuery to change a part of an image's source URL?
I'm working on a page that has a number of image elements that are all programmatically generated (but I'm not able to access the source code that generates them). And they all share the same class.
Let's say I have the following HTML:
<img src="https://cdn.example.com/pictures/01.jpg" class="photo">
<img src="https://cdn.example.com/pictures/02.jpg" class="photo">
<img src="https://cdn.example.com/pictures/03.jpg" class="photo">
<img src="https://cdn.example.com/pictures/04.jpg" class="photo">
Simple enough. But now, what if I want to point them all to a different directory? Like: /images/
So far, I've tried a few things, including this bit of jQuery, but nothing's done the trick so far:
$("img.photo").attr("src").replace("pictures","images");
That feels like it should do it because it's targeting images with that class -> then the "src" attribute -> and telling it to replace "pictures" with "images."
But I'm extremely new to using jQuery, so I'd appreciate some help.
What am I missing here? Any advice?
UPDATE: Huge thanks to those who provided answers and explanations — I really appreciate you helping a beginner!

In your code you simply change the returned string from
$("img.photo").attr("src") without doing anything with it afterwards. This will not change the attribute in your <img> elements.
This should do the job:
$("img.photo").each(function(){this.src=this.src.replace("pictures","images")})
Here I go through all the matched elements and assign the changed src string back to each element's srcattribute.
$("img.photo").each(function(){this.src=this.src.replace("pictures","images")});
// list the changed src values in the console:
console.log($("img.photo").map((i,img)=>img.src).get())
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<img src="https://cdn.example.com/pictures/01.jpg" class="photo">
<img src="https://cdn.example.com/pictures/02.jpg" class="photo">
<img src="https://cdn.example.com/pictures/03.jpg" class="photo">
<img src="https://cdn.example.com/pictures/04.jpg" class="photo">

Your code snippet won't work, because that's grabbing the src attribute of the first image, and changing it, but never setting the result as the new src on the <img> elements.
Try this
$("img.photo").each((index, img) => {
img.src = img.src.replace("pictures","images");
});

Change the src of HTML image tag to load image from a url using Javascript

Sorry to sound naive since I'm a beginner. I have been trying to load an image on my website with the image hosted somewhere else online and having a url "http://www.vapor-rage.com/wp-content/uploads/2014/05/sample.jpg"
I have an image tag defined as below
<img id="image" src="" alt="" />
Moreover my javascript function executes the following on a button click
document.getElementById("image").src="http://www.vapor-rage.com/wp-content/uploads/2014/05/sample.jpg";
The URL seems to work fine, but the image doesn't show up onclick.
Am I making any mistake?
Thanks for any suggestion :)

I could see that you are using " inside here, may be the HTML would also have " so that, it might not work. You can use the following code:
<a href="#"
onclick='document.getElementById("image").src="http://www.vapor-rage.com/wp-content/uploads/2014/05/sample.jpg"; return false'>Change</a>

I created a Jsfiddle, and it seems to work fine.
https://jsfiddle.net/ltlombardi/jvts4m3y/
var coco = function(){
document.getElementById("image").src="http://www.vapor-rage.com/wp-content/uploads/2016/02/Vapor-Rage-Small-Logo-new.png";
};
<img id="image" src="" alt="" />
Click here

using onerror for 404 for src different then img

hey so i'm trying to use onerror with img tag which will check if website is up or not. According to my research i have done onerror can be used to get status for website. So what i'm trying to accomplish is if website has an 404 it will return an image which red color however if website is up and running then it will show an green image. So i was able to come up with a code that does the trick but issue is i want to combine the code so it will only show one image at a time except two below is sample of my code any help will be apprecaited.
<img src="badlink" onerror="this.onerror=null;this.src='https://upload.wikimedia.org/wikipedia/commons/thumb/2/25/Red.svg/120px-Red.svg.png'" />
<img src="https://www.google.com" onerror="this.onerror=null;this.src='http://greensportsalliance.org/images/darkGreenSquare.gif'" />

<img src="badlink" onerror="onerror=null;this.src='https://upload.wikimedia.org/wikipedia/commons/thumb/2/25/Red.svg/120px-Red.svg.png'" />
<img src="https://www.google.com" onerror="onerror=null;this.src='http://greensportsalliance.org/images/darkGreenSquare.gif'" />

CSS/Javascript Lightbox

First I'll start with this: I am in no way shape or form a developer, coder etc etc. I'm just a graphic designer helping a friend with her website.
As of right now, I'm having issues linking up thumbnails to the full images on my lightbox call out - you can view the site at www.chrissybulakites.com
I noticed
With VOID:(0) being in every single one ... my thought process was that if I correspond 0 thumb with 0 full then 1 thumb with 1 full then 2 thumb wwith 2 full etc etc it would work .. it didn't.
Can somebody explain to me if I'm on the right path or what I can do to make this work.
Thanks
Rob

Have have two basic elements per image; the thumb and the full image. The thumb is using JavaScript to show and hide a div (kind of like a frame) to hold the full image.
The HTML on the page repeats itself a lot, you can probably solve your problem whilst removing some of the repetition. I'd keep all of your thumbs but on each one, add in a reference to the full image the thumb represents. As well as reducing repetition, it'll make it easier to update the page in the future as changing a thumb and main image is done in one place rather than two.
In the below I've added another part to the "onclick" to say update the src of 'frame' to be the full version of the thumb.
<img src="http://chrissybulakites.com/thumbnails/longshot_thumbnail.png" />
Then delete all of the large images except one, updating it so that the img tag has an ID of 'frame'
<div id="light" class="white_content"><img id='frame' src="http://chrissybulakites.com/images/longshot_full.png" /> <br />Actor Observor - Boston, MA Close</div>
<div id="fade" class="black_overlay"></div>
This will mean that as each thumb is clicked, it will do the light and fad bits it did before but it will also update the image being displayed.
Doing this for two images as a proof of concept I get this which works as expected:
<img src="http://chrissybulakites.com/thumbnails/longshot_thumbnail.png" />
<img src="http://chrissybulakites.com/thumbnails/actor_thumbnail.png" />
<div id="light" class="white_content"><img id='frame' src="http://chrissybulakites.com/images/longshot_full.png" /> <br />Actor Observor - Boston, MA Close</div>
<div id="fade" class="black_overlay"></div>

you need to give each full image div its own unique id like: id="image23". Then modify the onclick to refrence the corresponding id: onclick="document.getElementById('image23')...

The meaning of the function void() in JavaScript is "do nothing". This prevents it to load a new new page (or to open the thumbnail image).
onclick = "document.getElementById('light').style.display='block';
document.getElementById('fade').style.display='block' "
Says that when user clicks that item it will capture the element light and change the display to block it will also capture the element fade and change the display to block. The thing is all your images are wrapped in an element called "light" so the browser is opting to show the first one (instead of throwing an error).
There is plenty of fuzzy logic here.
Starting with the fact that you are loading all images (the high definition ones).
If you want my two cents (and you only want to get the results, as opposed to learn how JavaScript works) I would go with something like prettyPhoto that does it out of the box, in an easy and straightforward way and is well documented.
How to add prettyPhoto to your page?
Download the code and include both the Javascript and the CSS file's on your header.
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.1/jquery.min.js"></script>
<link rel="stylesheet" href="css/prettyPhoto.css" type="text/css" media="screen" charset="utf-8" />
<script src="js/jquery.prettyPhoto.js" type="text/javascript" charset="utf-8"></script>
Then put this code on your page
<script type="text/javascript" charset="utf-8">
$(document).ready(function(){
$("a[rel^='prettyPhoto']").prettyPhoto();
});
</script>
The docs say to put it on the bottom of the page but you (should) also put it on the header.
And then put the thumbnails with links to the actual images. PrettyPhoto will take care of everything else. Do note the rel="prettyPhoto[my_gal]"
<a href="img/full/1.jpg" rel="prettyPhoto[my_gal]" title="Caption 1">
<img src="images/thumbnails/t_1.jpg" width="60" height="60" alt="Red round shape" />
</a>
You can customize it further and should really read the manual here.

Make html valid if adding new tag for <img>

I have html img list like this one:
<img src="over.png" width="150" heigh="72" />
<img src="2_over.png" width="150" heigh="72" />
<img src="3_over.png" width="150" heigh="72" />
but these images are quite large. I need to make it that it will load one by one "like ajax". But I cant load it from javascript in the begining, becouse it is our cms desing like that and we will have big troubles if i will change it. So what i am planning to do is:
<img src="" width="150" heigh="72" path="over.png" />
and load it one by one with javascript and replace src, but the html will not be valid. Can any one give me any solution how to implement it with valid html?

If putting the data somewhere is your sole problem, HTML5 supports data- attributes, which let you add arbitrary attributes to elements. It remains valid as long as the attribute name starts with data-. For instance:
<img src="" width="150" height="72" data-path="over.png" />
This feature, while not "valid" HTML4, should still work on all browsers as unknown attributes on tags are simply ignored (and kept as is).
Though, you may also search for a way to do it only with JavaScript. I don't really understand how dynamically adding images could break your CMS.
Also, the alt attribute is required for images (make it blank if you don't have anything useful to write instead), and I'm not sure a blank src attribute is valid (you may want to put a loading image's path instead).

set a valid url in the image before you call the javascript function, for instance like this:
<img src="blank.gif" alt="Blank Image" width="10" height="10" />
And as for the "path" attribute, you could put the link to the image in the alt attribute. That way your image tag will be valid.
<img src="blank.gif" alt="myImage.jpg" width="10" height="10" />
Then you can use javascript to fetch the image path from the alt attribute, and insert it in the src attribute when loaded.

Those images should be thumbnails, correct? So you're downscaling large images via HTML to small sizes. The best way would be, honestly, to provide already downscaled images as thumbnails.

One solution is to have a small image loading image and set that as the source for all the images until your javascript has loaded the relevant image.
<img src="loading.png" data-path="over.png" height="50" alt="Over" width="150">

You may want to check out JAIL, the jQuery Asynchronous Image Loader:
While it may be slightly more than what you need, its got lots of options available and its less code you have to write!