I did something similar for my users' default avatars, randomly picking an image from a directory of a few thousand images I gathered and renamed sequentially. I just scoured deviantart.com for texture icon packs (usually 100x100px) that people seem to make for some reason, and threw them all in there. I spent a night quickly flicking through them and deleting anything exceedingly boring, too effeminate (majority pink/purple), or just lame. It hasn't gone public though, so I haven't experienced reactions yet.