I like to change my desktop every month. A couple of my favourite scrapbooking manufacturers provide free desktop calendars each month.
Basic Grey uses their pretty paper designs for their monthly calendars. I love the July one. Very rich looking with reds and greys. It's the one I chose for my work computer. Check it out here.
BoBunny also has a desktop calendar to upload. You can personalize it with your own photos. Follow the instructions to insert your pictures. Very neat. If you are a camper you will like the July desktop. Find it here.
If you just want pretty pictures for your monitors screen check out National Geographic. They have some stunning photographic wallpapers.
If you know of any others leave us a link in the comments below.