Mozilla and Mozilla Firebird both allow you to view a pages info including the exact url of any images called. This has been discussed to death in a previous thread located at
http://www.webproworld.com/viewtopic.php?t=268. You can not stop a person from copying something they can view in a web browser.
It did also occur to me that they wouldn't want to copy the images from your site anyway. If they like the design they would recreate it for themselves. Copying the image would serve no purpose.