Develop Reference

How are images downloaded on a incremental resolution basis?

Normally when you keep <img/> tag it shows from top to bottom while it is being downloaded from the web and looks very ugly but if you somebody's Facebook album and click on some photo you will see the whole photo is downloaded i.e the whole photo looks blurry/hazy and then as it downloaded from the web the picture goes clearer and clearer.
It looks so natural and as against the default behavior. How can this be done? I know it is Facebook and they can do anything they wish ;) but I need a brief overview of what they might have done to achieve this.

It's the image that is in special form - saved as progressive.
About progressive JPEG:
A simple or "baseline" JPEG file is stored as one top-to-bottom scan of the
image. Progressive JPEG divides the file into a series of scans. The first
scan shows the image at the equivalent of a very low quality setting, and
therefore it takes very little space. Following scans gradually improve the
quality. Each scan adds to the data already provided, so that the total
storage requirement is roughly the same as for a baseline JPEG image of the
same quality as the final scan. (Basically, progressive JPEG is just a
rearrangement of the same data into a more complicated order.)
More information here.

Interlaced jpg's do that. As far as I know, it's not code, but it's how you edit/save the image in the image editor.

How do you show pictures as fast as Facebook?

Can any of you help me to be able to show pictures as fast as facebook does!
Facebook is incredible to watch pictures at, because the pictures are kind of preloaded I think.
Often ved you view galleries on other sites, it is a pain in the a**, because it is so slow every time you change picture.
I think you need javascript to do it!?

Depending on your implementation, you could do this with some ajax and hidden dom elements.
Suppose you have a gallery with a slideshow.. You could insert a hidden dom element with the picture next picture of the slide show for each load. This would cause the image to be loaded. If you then were to use JS to insert that same image tag later, the browser would rely on it's cache rather than fetching it form the server since it already has that photo.
This is kind of a broad question but I think this approach would work. You would probally be better off not reinventing the wheel and seeing what Image prefetch librarbies based on JQuery or whatever are available to you..

Facebook compresses images to extremes. Try it yourself, take an image you are having trouble with and upload it to Facebook. Then check the size of the image, you will know why. Once I did a small test by uploading 17429 bytes image and it compressed it to 18757 bytes, a complete 7% increase from the original size!
At that compressed size, you can implement some sort of prefetch next image for display. Along with, I think, they have extremely good infrastructure.

Facebook uses Bigpipe, there is an open implementation in the works called openpipe
Bigpipe pushes the content to the browser when server stopped processing, so user will notice that it is faster.
It basically loads pagelets, when they are ready for the user, at the browser the implementation is Javascript based, and you must push the info to the client with your preferred server language.

First of all, facebook heavilycompresses images. Many other websites don't. Facebook also has a faster network than most other websites.
With the small image size, the client can prefetch the next image.

Preloaded would mean loading when the page is loaded, which is what happens with an <img> tag. No, it's simply because the file size is smaller.

If your wanting images to be viewed quicker on your site first make sure the images are decently compressed and aren't any bigger than they have to be. The amount of times I have seen websites using an extremely large image scaled down to fit in an element 5 times smaller is just ridiculous.
You can check out these sites that has many implementations and links on how to pre-load / pre-fetch images (css, JavaScript, ajax)
http://perishablepress.com/press/2009/12/28/3-ways-preload-images-css-javascript-ajax/
Since your question was tagged with 'jquery' here is one just for that.
http://engineeredweb.com/blog/09/12/preloading-images-jquery-and-javascript

What is the most efficient way to store an image? HTML/CSS/JS

I am going to have a lot of images and trying to find the most efficient way of storing these images to keep the page snappy.
So far I have thought of just the two ways: load with javascript eg picture = new Image(); picture.src = "file.jpg"; and append / remove to the page as necessary, or load into <img> and set display:none.
Are there other options? what is considered the best way to do this?

The best way for a photo gallery (if thats what you are building) is usually to have several sizes of the images, at least two:
a smallish size that is highly compressed and thus have a small footprint: this is the image you load into grids and display in a page where there are multiple images
a larger image with lower compression and higher image quality - this is the one you show when people want to see details.
Since people most often come to the detailed image from a page where the small/fast loading version has already been shown, and thus is already in the browsers cache, you do a little trick and have instant photos, without preloading anything.
It goes like this:
On the details page you show the highly compressed small image in an image tag that has the dimension of the larger detailed version. You then load the larger detailed version in the background using new Image() with an onload event attached that changes the source of the image tag with the small compressed version to the large detailed version.
It looks great, works fast and users will love you ;)
PS: the best way to store images is the browsers cache, not js or the DOM, so if you truly wish to preload images, which is generally a bad practice (tho it can be necessary sometimes), make the browser fetch them for you in the background by including a css file that references them in styles that aren't applied to visual areas of your site.

I'm not sure about "efficient", but the most logical way would be not use the JavaScript to load an image (useless if you have JavaScript disabled) or to set the image as hidden via the display property (likewise, and the browser will probably just load the image anyway).
As such, a sensible suggestion would be to use boring old paging and display 'n' images per page. However, to bring this up to date, you could use "lazy" (a.k.a. "deferred") loading and load additional page content via Ajax as the user scrolls. However, it's key that this gracefully degrades into the standard "paged" behaviour if JavaScript is disabled, etc.
The perfect example of this in operation is Google's image search, and if you search here on StackOverflow you see a discussion of possible implementations, etc.

It's better to use javascript the way that you have it and then add it to the DOM as you need, as opposed to first adding it to the to the DOM and then hiding it because DOM manipulation is much slower and you may not use some images

Javascript - Visual Representation of Element as an Image

This is a long shot but I've seen things which might make it posssible.
I have a div, which is filled with images. Album covers if you must know. And I want to allow users to download this as an image. So they could use it as something like a desktop background.
So is this possible? Get this visual representation of an element and display it as an image?

Basically you can't do that. At least crossbrowser. But if it is not critical. You can try <canvas>
check here http://www.nihilogic.dk/labs/canvas2image/

Assuming I understand the question...If you know the position of the images in the div, you could concatenate the images together server side into a single image. Then just have a button users can click on that will call the function to assemble and download this image.

From what I understood from your question you can use an
img
tag for this. The user can view the image in the browser and can save it to their hard drive.
from your description each of the image inside will be a different album cover, so combining these into a single image won't a good idea.

You could possibly do the rendering serverside. By this I mean that you could generate the HTML and kind of "screenshot" it on the server. The result would nearly always be at least slightly different from what the user sees, but depending on your requirements it might be enough.
There are various tools to do this, for example wkhtmltoimage, which is a sister project of wkhtmltopdf and can be found at https://code.google.com/p/wkhtmltopdf/

Editing a image in the browser using Javascript then saving the image in Google App Engine

I would like a user to be able to edit an image that I serve, using Javascript, and then upload that image back into App Engine.
Original image is stored as Blob in DataStore.
Image is served (currently) just using
Currently user can only view the image, but i want user to be able to edit the images (adding text and lines at most, no complex transformation needed).
User can save the image back into the web application.
What is the quickest way to do this?
Will it be easy to integrate with 3rd party javascript image libraries (which has the editing tools already)?

Your starting point (and minimum requirement) for line art (read: not pixels but as you say: text and lines) will be the HTML5 Canvas Object:
http://www.w3schools.com/html5/html5_canvas.asp
Depending on your use case you may want to 'underlay' the pixel image to be decorated.
For saving refer to
http://www.tutorialspoint.com/html5/canvas_states.htm and
How to save HTML5 canvas?
If the end results shall be pixels, Canvas2Image might by for you.
Sorry to just throw general links at you, but since you asked in a general way. Hope this helps.