The Lightbox JS script by Lokesh Dhakar is something I've been looking for for a long time and the way to go when it comes to previews and full size displays of images, without breaking usability (shitty pop-ups) or accessibility (nothing working at all when JS is disabled).

It allows displaying a small image preview along with the functionality of laying the belonging bigger image over the current page with a nice alpha blended background around.
You might say "if the user will disable their JS, they're not going to see anything, eh?".
Well, no. That's the great deal. A user (bot, whatever) without JS enabled will see a link to the bigger version of the image. No pretty blended border, but still no broken function.

Nice one: Lightbox JS.