If you want to take an image from a site and have the nouse its pretty easy. Even with right click disabled, you can view source or edit in frontpage and the image location will be displayed.
Images are cached on the viewers PC. They can trawl through the cached files.
Watermarking will provide provenance, but not stop copying which is an impossiblity anyway, however disabling right click or similar will stop many less educated internet users.
www.thesun.co.uk seems to have a very elegant right click solution, works on text, not images.
A copyright notice should also be prominantly displayed on your site.