Your only real protection is a watermark if that's acceptable. Embedding images in flash only adds a few more steps to extract them (swf decompilers). Where a watermark wouldn't be appropriate, there's a few layers of protection:-
-Disable the context menu for images with JS
-Hide the images when window/tab looses focus (to launch another app to take a screenshot) with JS
-Display images with JS enabled only
-Hide images behind transparent slices
-Serve images with session based scripts (impossible to leech or display outside of the webpage)
-Embed a swf that constantly writes garbage to the clipboard (overwriting screen grabs)
-Use image slices and glue together on the page (forces user to edit images together)
-Use headers to stop webpage caching (saving image from browser cache)
Despite these current methods of protecting images, FF's Page Info->Media Tab->Save image as is hard to beat.
Here's a link to a web page designed to use most of the image protection methods
above. I'll repost it for a week or so. It was originally done in '07 to explore exactly what you're looking into. Nothing has changed in the last 3 years. This is about as good as image protection gets unfortunately.
100% sure-fire way? Don't digitize your images or publish them to the web.