(starting new thread, as the exhibitionist one doesn't entirely encompass this topic;-)
I went ahead and hacked a little javascript to draw clickable image boxes. To see it (it is completely invisible to everyone by default, except for a template that adds a new category to the image description page), look at [[Template:Imagebox]] and follow the instructions.
To view the only example of this (my own picture, as I didn't want to vandalize someone else's;-) look at [1]. It marks the bee in a semi-transparent box with a red border, with the name of the category it links to ("Bee") written on it. If you click on the box, you go to that category.
Notes: * This is just a demo. If you want it for real, gimme feedback! * This is the very first halfway working version. There's no show/hide toggle, no "box drawing" function, nothing. * Almost everything about this can be changed. * No, I won't do circles or ellipses ;-) * For me, it didn't work so well together with my other commons javascripts ;-) If you have problems and use other javascript, surround the other JS /* like this */ for testing purposes.
Cheers, Magnus
[1] http://commons.wikimedia.org/wiki/Image:Valeriana_officinalis_2007-06-02_%28...