When you send a pic in you release the right to own the photo.

That gives them the right to print and make money on your photo.

You don't win the 100 dollars but you do get the chance to buy a book, They just have to award ONE prize in order to keep from being called a "mail scam".

Check the entry for and I bet there is a line that says something like...

By entering your photo in this contest you release all ownership rights and let ------------ use it in any fashion they choose....