I just ran across this website (Planetary Photojournal) just a little bit ago, and Holy Cow, are there some wonderful image son there. That is where I retreived this beauty of Saturn.
With thousands of images on all of the planets, and information on the equipment used to make the images – there is a wealth of information on this site. This is a really nice image of Mars from the Rover. The galleries are organized well, and the site is searchable too – so it’s pretty easy to find images that might interest you.
There are a ton of wallpapers for download as well as audio, video, and other media as well. Check it out – it’s worth poking around for a bit.