You can't. There are scripts to block people from right-clicking on an image to select Save As, but they are easily defeated. Even in a popup window, in IE a simple Shift-F10 will get them the option to view your source where they can get the direct URLs to your images.
Your could turn them into Flash animations, in which case a screen capture can be done and they can crop it out using an image editor.
If you don't want someone stealing your graphics, your options are basically to make them of a low-enough quality that nobody would want to use them directly, or not put them online at all. An aggressive legal team scouting for pirates can also be effective, albeit costly.