There are lots of utilities available which are dedicated for the function of changing the desktop wallpaper at periodic intervals of time like Desktop Magic 1.1 , Smart Wallpaper , Wallpaper Desktop etc.
Out of all the utilities above I found Any Wallpaper the best and simple to use and it also consumes less memory as compared to other desktop wallpaper changing programs.
Any Wallpaper – An automatic desktop wallpaper changer
As shown in the image above you just need to click the Add button and browse the folder containing wallpapers.
It offers various options like how do you like the wallpaper to get positioned on the desktop.
As shown in the image above you can choose the wallpaper position as auto, autofit, fit horizontally, fit vertically, center, stretch etc.
You can also use the toggle button to switch in between Yes or No option to set images in the sub folders as desktop wallpaper, and after installation it automatically adds itself to windows startup( shown in the image above ).