Some people like to save sites for offline viewing, especially when their Internet access is prone to interruption or they anticipate being offline for extended periods. If protecting images are your main concern, watermark them with your name and URL. It won't prevent downloads (nothing will), but watermarked images are less attractive to pirates. Also look at digital watermarks for tracking your online media.
About Digital Watermarking and Watermarks | Digimarc
Nancy O.