Monitors can only display at 72 DPI, so if you want good quality backgrounds, you have to look for ones that aren't saved in JPG format. Look for TIFFs instead. The JPG compression is often what makes the picture look lower quality. www.macdesktops.com is where I go for backgrounds. Lots of selection there.
http://x.themes.org has a great selection of pictures specifically scaled for monitor resolutions.
NASA's astronomy picture of the day http://antwrp.gsfc.nasa.gov/apod/astropix.html has an archive that goes back for years, some really gorgeous stuff there. Mostly not scaled for monitor resolutions, you can get quite high-res versions of many pics.