I have a suggestion for a theme: Creative anachronism. Any image that is wildly (and creatively) out of it's time frame would be fair game.
A classic example is the dogfight scene in Final Countdown, where F-14 Tomcats are mixing it up with A6M Zeros.
A variation on this theme would be alternate history, where a scene is depicted of an event that never happened and would have significantly altered history.
For example, a successful hostage rescue mission in Tehran.

I like the concept as well. It sounds like it'd be fun!