I am constantly looking for unique and off the wall images for my desktop, whether it is to convey a mood I am in or to freak out co-workers sitting next to me ;) Here are a few great sites with some real interesting illustrations and photographs on a wide variety of topics and themes.
Wallbase - This site is great! You can search for hundreds of images using their indexed database. The images are scraped from the net regularly. I am always finding cool visual eye candy on here.
Vladstudio - Russian artist, Vlad Gerasimov created the site in 1998 to show off his talent. The site offers a variety of images in a number of categories. His unique style of imagery is something I haven't seen before on other sites. I have several monitors displaying his creations.
Deviant Art - Primarily a website for budding artists to show off their talents. This site also offers a collection of wallpaper that is quite good. The website also offers images and backgrounds for web design. I use this site often as a resource and for inspiration.