Here's a fun little tutorial by Ged from Iconfactory, showing how to easily use icons (exported to a certain size) as contact images for the iPhone. His example is with the Dino-o-Matic icons from IF, which are (cheer!) free this week, but the fact is that this is a great idea for almost any contacts that you don't have a readily available picture for. You could use any icon you like, use Pixadex to get it in the right form, and then plug it right into your Address Book.
As Ged notes in an update, while this is perfect for iPhone, it could be used for anything, including just Address Book itself. Because why say with a crappy, out-of-focus picture what you could say with a beautiful icon?