Copying Graphics - What is Fair Use?

it0 asks: "I'm writing a web application and since I can't create good graphics, I'll be ripping them from other places on the web. The decent thing to do would be to ask permission, and at least specify where you got the graphic, however I don't see this happening much on other websites. Here's an example: I copied a trashcan icon that seems to be used by everyone and nobody seems to specify its original source. What about wallpapers? I've see a lot of models without references? I've also seen a lot of images that imitate the Windows GUI, and I've yet to notice anyone getting sued! For those interested, here is more information on the subject."