Hi all,
In order to reduce confusion, personally I would think it better to refer to it as a image usage license in this case. You need to "license" your friends to make prints in small quantity for personal use.
A "release" is more often a term that's applied to models and property. This isn't something that is needed or relevant in this particular situation.
You actually don't need to prove anything. Just write up a usage agreement that permits them to make the reprints for personal use, and sign it. It can just be a letter that says something to this effect. They should keep it with the originals in case they ever wish to make more prints (whether they be from digital files you have provided or scans made from prints you provided).
Actually I'm really glad to hear that Walmart is restricting reprints in that way. That's just as it should be. All reputable printers used to do this, most probably still do. If there is a signature or a copyright notice, they shouldn't make prints without the owner of the copyright providing written permission.
If the printer infringes upon the copyright by ignoring the signature or copyright mark or embedded copyright information and going ahead and making reprints, they are opening themselves up to lawsuits. (A person making scans and prints at home is every bit as subject to lawsuits, by the way. Their pockets probably aren't as deep as Walmart's, though.)
If there is not signature or copyright notice on the front or back of the print or embedded in the digital file, they could probably successfully defend themselves with an "innocent infringement" argument.
That's no reason to leave off the signature or copyright notice in the future, though! Just the opposite, in fact!
Now, I'm not an attorney, nor did I stay at Holiday Inn last nite.