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I'm also thinking about stuff that has been built upon. Wanting a brick back that someone has lying on his desk is one thing - wanting a brick that is incorporated in the wall behind the desk is another thing (possibly). Thanks!
If you ARE paying them, then your site should make it clear that you own what you've paid for (if it is not made clear before they submit, then you DON'T get to own it!
A better way is probably to say that they are giving you a "license in perpetuity" - and make that user friendly by emphasising that they do get to keep the copyright (but they can't stop you using it later!).
The way you phrase your agreements is very important, and (of course!), I can't help you there - but there's plenty of info on copyright and licenses out there.