If like most folks that take their own pics you edit them before posting them you can watermark them very simply in photoshop. If your photos are your products then you might want to visit
http://www.digimarc.com/ and check out their solutions.
Alternatively you can make sure your online pics are a) the lowest resolution you can use that retains what you are attempting to convey and b) use the web palette of 256 colors. At 72 dpi with 256 colors very few pics are going to display properly at any size other than what you save them for. Makes them worthless to rip off for prints and saves on file sizes as well.