Although you don't see yourself as a Pro, the Copyright law still protects you. The law states that once the work is created (picture taken, painting painted or music written, etc) it is copyright protected for 70 years (Pretty sure it's 70) from the say you die.
It's there to protect you and I am actually glad to see that Wally world turned them away. Give them a letter saying that you either give up the rights to the works, or you give them permission to print them. As for being able to tell who took them, a lot of information is stored with a digital file, you could embed your name into them and then they would be able to match the name to the letter.