Welcome to TalkGraphics Elgin. I always add a copyright line to the image but this does not stop the image from being downloaded and added to someone else's website.

Encrypt Web Pro (http://www.allynova.com/) can stop a lot of right click, drag and drop, and other attempts to copy content. It is not fool proof and as has been pointed out, Print Screen will copy whatever is on the screen at the current resolution. But as they say about locks, it will keep the honest people from copying/stealing your content. You can even add a pop up message telling the visitor that the image is copyrighted and to bugger off. Or words to that effect.