In Windows 7, you can make custom Aero Colored windows themes with a slideshow background which changes the background on your computer for every 10 seconds depending on what you set it to.
All you have to do is right click your mouse on your desktop, and pick personalize from the right click menu. You can now adjust your backgrounds orientatation, how many backgrounds you want, the amount of time each background picture shows for, Windows Aero color for the taskbar and windows, system sounds and screensaver settings.
Once you are done custumizing your theme. You can save it to your computer by clicking the save theme link on Windows personalization. You can also save your current theme for sharing online and offline by right clicking your current theme and pick “save theme for sharing”.
If you don’t feel like making your own themes, you can download some themes from http://windows.microsoft.com/en-CA/windows/downloads/personalize?T1=themes .
Wallpaper sites I like to visit for my Windows background picture downloads:
http://www.deviantart.com/
http://customize.org/
http://www.animewallpapers.com/
You can also make your own wallpaper with MS Paint, Gimp, Photoshop, Irfanview, and many other tools. I recommend saving your wallpaper files as JPEG or PNG files.