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.