If you're asking about digital photography then you have some good answers but if you're wanting to print the photos out or are using film based photography then maybe this will help.
Studio portrait photographers use a gold foil hot stamping machine. The process uses a hot brass die that is pressed onto a film of gold foil on top of each photo. Studios use a discrete copyright and their name across the corner of the photo (usually bottom right). This is the same process used to put the title of book on the spine or the cover. Wedding photographers use the technique to personalize wedding albums with the names of the bride and groom on the cover (and the studio's name discretely in the corner).
The equipment isn't cheap however. You can even buy them on eBay.
Do a Yahoo! or Google search for "foil stamping" or "hot stamping" and you will find lots of information.
I used to use a tool that printed my logo on the corner of each print while it was still on the enlarger easel. It used a small piece of camera film. You took a photo of your signature, name, logo, whatever and when the film was developed you had the logo clear and the surrounding area black (a simple negative). The film was cut to fit the front of a little black plastic box. Inside the box was a small flashlight bulb wired to a switch and a battery. You placed the film end of the box (which was open to the film, of course) over the print and held the switch on for a few seconds sufficient to expose the paper with the logo. This obviously wouldn't work on a photo with an already black area in the corner but was great for portraits and wedding photos. I haven't seen one of these in years but it would be easy to make one.
Pros who are submitting photos for publication often will rubber stamp the back of the photo with copyright info giving limited publications rights with their name, address, phone, etc. If you're using RC paper not all rubber stamp ink will work without smearing. On fiber paper you have to be careful to let the ink dry before stacking the pictures together.