That was my original (long winded answer) many years ago and I'm foggy about the whole code now as I don't develop for android at all now.
However, if my memory serves me well, you need an image saved in your drawables folder. I called mine 'bg'. It was used as the default image when the app started and was also used as a roll back image if the code couldn't resolve an image that the user chose (e.g. it was deleted later).
I hope I'm correct, but it was about three years ago I wrote that answer.
Good luck.