If you find that your Windows desktop slideshow, which used to work fine, does not work anymore, you may want to check out this article.
First make sure that no installed software is preventing changing of the wallpapers. Also ensure that your version of Windows 7 has this feature.
Windows 7 Starter and Windows 7 Home Basic do not support changing of the desktop background picture and consequently support the slideshow feature; so you might want to confirm that! But sure there is a way to change your wallpaper in the Windows Starter and Basic Editions too!
Having done that, click on the Power icon sitting in your notification area and select More Power Options. This will open Control Panel, Power Options applet.
Now from your current/selected Plan, click Change plan settings and then Change advanced power settings.
Next in the Advanced settings, expand Desktop background settings and then Slide show.
Here from the drop down menu of each option, ensure that the appropriate options are checked.
If you cannot change the desktop wallpaper or if you cannot set an image as desktop wallpaper, see this.