While you're thinking, consider placing a transparent gif file over the images you want to protect. Visitors won't be able to access the protected image via right click but, since it's on their computer, they can merely look at their own cache files to "capture" the protected image.
If you want to use JS or CSS to change the image on a mouseover, JS can be turned off and CSS must be read by browsers so they're of little benefit (in protecting an image).
Using "slices" won't help either (not even if you change the scale and use JS/CSS code to resize and reassemble) because a visitor can read the code you're using and do the same thing.
In other words, if you don't want others to use your image, use a good watermark (difficult to remove but no guarantee) or don't put it online.
WARNING: Pontification follows
IMHO, the Internet should be free for original content. Humans ruin that concept by breach of copyright notices (piracy) and there is little (actually, nothing) you can do about it. Therefore, annoy the pirates by making it impossible to steal your bandwidth with your images (embedding your image in their website with a link) via mod_rewrite and, if you have copious spare time, try some of the above referenced "tricks" (especially the watermarks) with the knowledge that nothing will stop a determined pirate from saving your image and using it elsewhere.