As much as I love the picture of Pericles giving his oration (used in my nearby graphic of PD5), that one is probably already overused, and I really need a new source of public-domain pictures from the Greco-Roman period. (Other imagery that works as well is also good, but much of the public domain stuff is going to be "classical.)
I use Wikimedia as one source, but I am sure that there are many books on google docs or other online sources which have good illustrations and are now in the public domain. For example, I've personally seen that many older texts on Latin and Greek grammar frequently have good illustrations. If anyone knows of examples of such sources we could use for future graphics I would appreciate your posting links here so we can meme away without fear of copyright issues down the road.