Don't put them on your website. Seriously, anything on the web can (and
probably will) be copied. You can use watermarks or lesser quality graphics
but even they can be "borrowed".

Are you using version 2003? If so have you right clicked an image and tried
to save it?

David Bartosik - [MSFT MVP]
www.publishermvps.com
www.davidbartosik.com

Fundamentally, there is nothing you can do.  The way a browser works is it
downloads all the binary files from a site along with the text and displays
them onscreen.  By necessity, in order to view a page your must download the
page's contents, images and all, to a local memory cache and accessing it
for "archiving" that info is relatively easy.

Your best bet is to watermark the dickens out of your materials.  Or like
someone else suggested, don't post them online!