I sometimes save pictures to my hard drive from e-mails people send me of great photographs. They're usually maybe 10 pictures that are sent just to look at. Some are really great.
At present that's my only source. I don't change them that often anyway. I may in future try some of the suggested sites.
http://www.fotocommunity.com/travel: It's a Photo site, mainly I use his pictures and set as wallpaper. Once I too used Caedes, and now sometimes http://www.trekearth.com/gallery/
probably the best way to find wallpapers, imo. you can even specify a dimension for the image in Google search so it makes the job easier.
I am waiting for the site to get out of alpha stage , till then i am using http://thepaperwall.com/index.php