Here is a common way we used for years:
<a href="image-3200x2400.jpg" target="imageframe"><img src="image-320x240.jpg" width="320" height="240" alt="Click for larger image"></a>
However this method has never been very elegant, and many authors today rely to some JavaScript or other client-side method to open the image in a small window, or even as a layer over the page where the original image is showing. [Use-Case]
I do like this idea:
<img src="image-320x240.jpg" fullsrc="image-3200x2400.jpg" alt="Click for larger image" width="320" height="240">
I would image a browser opening the 'fullsrc' image as either a small window or a temporary layer over the original 'src' image, without affecting the original page directly (thus not breaking layout or anything). [Implementation]
Off-topic:
I propose a new HTTP feature, so the http client can request a certain image size. This would be also useful for movies.
HTTP is a transport protocol. I always thought it should only be allowed to do as little as possible to accomplish its task. I don't think this task would be appropriate for the protocol (resizing images on the fly is somewhat resource intensive). However, this is easily and actually often done by some server-side script, an example being
GD in PHP[/quote]