Why not use the existing category system for tags, and
JavaScript for
"rollover"?
http://commons.wikimedia.org/w/index.php?title=Image:Valeriana_officinalis_…
Yeah, Magnus' contraption is already usable on commons. It just hasn't had
its breakthrough yet. I find the limitation to categories to be a
hindrance. I'd prefer arbitrary Wikilinks (or even multiple language links
to Wikipedias per tag). This would dramatically increase the number of
taggable objects per image and make tagging more attractive IMO.