How can I configure my phone to show random background wallpapers from my pictures galery?
You could try an app like WallSwitch, which can be set to change your wallpaper every time you unlock the device or on a timer. I don't know that there's a way to tell it to pull from "the gallery" in general, but you can just specify that you want to pull from the folders your gallery images are in (works out effectively the same).