This would be a great addition. It's very counter-intuitive as it is now... I've often wondered about some of those games, but usually don't want to get sidetracked from what I am currently looking up.
Pehaps an incentive would be bandwidth savings. When I see those pictures without tooltips I have to click each one (I don't want to, I have to!) causing your server to have to serve me up 4 full pages complete with everything on it. A tool tip would save the network traffic and (my) time (although some will click the pics anyway).