How do I rotate the pictures of my iPhone/iPad wallpaper every few minutes? I want the wallpaper on my iPhone/iPad to rotate/change to a new wallpaper every few minutes, so that I won't get bored seeing the same picture over and over again every time I'm on my phone. 

Are there any free iPhone apps which allow me to do so?
There are lots of ways to do this.  First Jailbreak your iDevice.  Then once your device is jailbroken look up winterboard.  install winterboard and find tweak option for changing wallpapers.