Not to quibble over semantics, Martie, but publishing images on your Web site doesn't make them "public domain" - just public. You still retain the copyright, and control the images, as I'm confident you're aware.

I agree with the other suggestions regarding model releases. The approach can be as simple as, "I'd like to put our results up on my Web page. If that's OK with you, would you mind signing this model release?" Although we probably have the right to use our images for self promotion under the law (at least in the U.S.), Web publication can push the question into the gray area. Thus, having a model release solves the problem. The "standard" form can also be modified to include non-commercial usage provisions, if that makes your subjects more comfortable.