Are you still going with packaged themes in the repository? I remember reading a few people's debates about how to distribute them. Or are you assuming people will know how to do that (or work that out as a seperate problem and are just looking for organization?)

If so I think a tag based solution would work better then hard categories. Maybe use a hard category for the dominant theme (e.g. nature, vehicles, women, etc..) then have tags people or the artist could add. Of course then the problem is naming said categories, and then what would most average users think of when they're looking for a specific image. And you don't have the ability to do a card-sorting study =p.