For our thrifty wedding, my husband's very talented cousin, Sara, served as our photographer. She gave us the photos digitally so that we could email, print, and post them whenever we wanted. When I took the digital media to the Wal-Mart photo center to print a few photos using their instant service, I was asked three times by the attendants if the photos were professional, then told that if they looked professional, I'd have to get a release signed by Sara in order to get my photos.
I asked what factors would used to be determined whether or not my photos were "professional," and was told it would be up to the attendant's judgement. Sure enough, he decided that the photos looked professional and didn't want to release them to me. Wal-Mart does have a standard release form. If you intend to print photos there, I'd suggest having whomever took them fill out the form in advance, then bring it with you when you go to get photos. You may be able to avoid hassle later.
As a professional photographer, I appreciate the fact that Walmart tries hard to honor copyright. Most digital images are encoded with metadata and professional photographers generally have their copyright information directly encoded into the file. A professional photo lab would be able to pull up the metadata and see quite easily that the photos are copyrighted.
