The copyright symbol. ©
To help protect my images on the web I put my name, the year I took them and a copyright symbol on them before I put them up.
The easiest way I have found to do this is on a separate layer. Then to get the © symbol use Option + G [mac], or hold down on the Alt key while typing the 0169 from the numeric key pad [windows] or hold down Ctrl and Alt at the same time and press C (Ctrl+Alt+C). Mac are easier at this. You can always type a copyright the simple way like this: (c).
This may not stop someone from taking your images but if a honest person would like to contact you to see about using your images it sure make it easier for them to find you.