On sites like Flickr you can display a little icon showing how you want your image to be used, you can choose that people can use it freely, can derive things from it freely, that they have to acknowledge you or not, that they can't use it at all unless they pay up etc etc - lots of different combinations. I have to admit to using people's (Flickr) photos in artwork I've been doing - but it's just personal stuff, I'm not showing it or getting money for it. And if you alter it substantially you're usually ok. Straightforward reproduction on someone's website though, that's a different matter I guess (most photos on the web wouldn't be of a high enough resolution to actually print out and sell?). I think on Photoshop there's a watermark adding thingy, and there's bound to be some freeware somewhere that'll do it too? Anyway I think I'm saying that I think it's down to the individual to put the watermark on, I think it's a shame to have every photo marked if the contributer isn't really that bothered?