To get back to the picture's being used. This is not different in any country I think.

Any picture on the internet has copyright protection, even if the owner does not mark the picture. If you took the picture and you put it on the Internet, nobody else but you can use it. Unless you give permission.

Because we agreed on the caption poll, implicitely this is thaught in. But it's not mentioned in the rules. So in next games I would at the line:
"Each participant in the photo contest gives his permission to use his pictures in the Caption contest/poll and only in the caption contest/poll. If the person wishes that his picture is not used in the caption contest/poll, he should mention this when he posts the pictures."

Personally anybody can use my pictures, except for commercial purpouses. Of course if it's used by someone else I like it when my name is mentioned. But for the caption contest, I don't really mine that they are being used.