I like the idea. The general area could be an intersection and the mystery picture a certain brick stone in one of the buildings at this intersection. Or anything else, just use your fantasy. THe only question I have:
Once the first person finds it and logs the coordinates, what is there left to do for the next person? You have to treat it like a virtual geocache: email the coordinates to the waymark owner and log your visit only after the owner responded.
Count me in...