Facebook image taggin

I was curious about this myself.  My best guess would be that it would be like any other tagging solution, but you would need to be able to capture/store a x/y location relative to the dimension of the image.  I believe most tagging solutions require a many to many relation ship where the item tagged has a many to many relationship to a tag, in this case the user.  I believe the join table will be the best place to add this x/y information.  The most difficult part (which I haven't wrapped my head around) is the capture of the x/y location.  My best guess is to always scale the images to the same height and width and when using javascript capture the location of where the user is clicking.

This is my best guess, but I'd like to hear what someone else has to say.