Quote Originally Posted by Serious View Post
Exactly what does this add-on do?

Any place I can see a demo?
Yes :) See a product in action (click the "larger image" link) or indeed, click any of the size guides and you'll see the effect.

You can, in fact, apply it to any image. Here, I have embedded a Picasaweb image gallery, and an using this plugin to show the full size image. Note that you can also click through the large images with the left and right arrow.