I'm a bit late to the party, but here's my 2 cents:
For the legal part, that depends on what was written in the contest legal text and what the contestants agreed upon. Perhaps there was a clause that gave all copyright to you. In that case, you're clean. If not, then probably you need to get a permission from them.
It also depends on what you "steal". If it's just conceptual ideas (icon here, checkbox here, layout, etc) - I think that's fine. But whole pictures, icons, texts, etc - that's not so good. Still, IANAL (I Am Not A Lawyer), so don't take this as a legal advice.
Now, for the ethical part. What feels right to me is this - it's OK to make contests, but they have to be made in a fashion that doesn't abuse the contestants. That is, a contest of "make the whole thing for me and the best one gets paid" is obviously an exploitation. But "here's one small part of what needs to be done; show me how you would do it and the best one of you gets a big juicy contract to finish the rest" - that's totally fine. This is like asking them for a sample of their work. You should however make sure that the part you're asking them to make indeed is small. Like, it should probably take the contestants no more than some 4-16 hours to complete. The less, the better. It should be enough to allow you to evaluate their skills (that's the point of the contest after all), but not so much that they would feel like they've wasted loads of time for no return.
Alternatively, you could offer all contestants that meet some minimum criteria some fair compensation. Be careful with the criteria though so that you don't get abused. And, of course, this depends on how much cash you have to burn on getting the perfect candidate.