UPDATE [02-28-12]: It sounds like WebP-Mux is the answer to creating Sprite Stacks, and it should be arriving in the next few months to Chrome!
This is a proof of concept of a PNG Sprite Stack; multiple images embedded in a single image "blah.png". Which are then replaced with the #hash target once the page loads. It is not a great POC, because it makes two requests, once from the IMG tag, and once from XHTTP... also, it requires the browser to support long base64 strings, but it works;
This was brought on by some discussion at the Stuff in my head blog about a new web image format Mister Morris has proposed; allowing you to package multiple icon sizes in one file. This would allow for the developer to render the appropriate sized image, no matter what device display size was being used.
Simurai made an interesting recommendation to store the files in a .ZIP, renamed to .IMG, and allowing the programmer to retrieve the images by simply calling; background-image: url(sprite.img[search.png]);—this provides the following benefits;
The PNG Sprite Stack (139kb) was created by combining multiple files using cat a.png b.png c.png > output.png. All images were optimized with ImageOptim before being combined (if it was optimized afterwards, it would strip out b.png/c.png). There is no optimization of the color indexes after the image was created (this would be built into the format).
With the right folks backing a project like the one proposed by Mister Morris there would be a lot of potential here, for servers, programmers, and users alike.