As an OpenGameArt, lack of tags is one of the main issues I see with art submissions, making them invisible to the site search.

I have two suggestions that should help this:

1. Display a list of 100 art item links with least amount of tags, sorted from least to most tags, make this list available to moderators.

2. Allow users to add (suggest) tags to art. The following are mockups of how this could be implemented UI-wise.

Mockup 1: adding & removing tags one by one

Demo: editing tags of your submissions on open clip art library

Mockup 2: text area tag editing for suggesting additions/removal as a user

Mockup 3: moderation interface form for moderators