Cassie:
Download Webshots. It's free and will do what you want. I love it. It will do both screen savers and desktop wallpaper. For wallpaper, you can set one pix, or have it change daily, weekly, every 4-hours, etc. It will also put a calendar on screen if you'd like.
The screen saver has numerous options to it also.
You just put the photos you want into a 'collection' (you tell the program what photos you want and where they are), and tell the program what to do with them.
Check it out at Webshots.com