Jim Bumgardner, better known as KrazyDad, does some wonderful stuff. He does things with Flickr, he does things with Flash, and it’s all great fun.
This is his Mona Lisa, for example, made entirely from Flickr photos with the tags ‘mona’ or ‘lisa’. Click on the image to see some more. And make sure you know about possibly the most important button in Flickr which is the little thing labelled ‘All Sizes’ just above a photo. You really want it for these! You can get some of them as posters, too.
Go to his site and browse around. It’s worth it.