Hi,
It's not a "release" per se. It's a limited assignment of copyright usage privileges using an "image usage licensing agreement".
Possibly more important than the permissions you do give, the agreement should also state the restrictions you impose and the permissions you don't give.
You do this by stating first that limited rights are being given.
Then you detail what is given, for example, that the client (by name) is permit the making of prints for personal use (either themselves or through printing services), the copying of files in low resolution and small size for personal use online such as for email attachments. These permissions might be given in unlimited number and for an indefinite period of time, or limited in either respect.
But, you should limit this to personal use and not allow sale of or allow any transfer of copyright to third parties, for commercial or any other purposes, in print or in digital form, worldwide and indefinitely.
I think many would also limit derivative works and manipulation or alteration of the original images, too.
Finally, make it clear that you retain your copyright and full control of all rights, along with your contact information.
In addition to something printed on your letterhead as Stephen suggests, I would also put a .txt file on the CD or DVD, with the same info, or a .doc copy of the letter, or a pdf. Anyway, just some common file format most computers would be able to open. That way if the printed letter becomes separated from the CD/DVD, there is info to contact you, and possibly enough for them to just go ahead and make prints.