I have a picture saved in My Documents that I would like as my screensaver. How do I do this?
The simplest way is to go to Display Properties - Screensaver tab - select My Pictures Slideshow from the dropdown and then go into Settings and customize it as you want. Of course, you need to put all the pictures you want in the screensaver in the My Pictures folder. (This is for Windows XP. Vista and 7 should be pretty much similar)
You might want to check out Instasaver too.
I would also recommend my own post on The 10 Creative Ways To Use A Screensaver