Have you done a google search for painted abstract backgrounds?
Well.. if you can't paint (which I know I can't, you can ask my Design 1 Instructor from college, and she gave me a A in that class.. I guess she felt bad, I tried so hard) or have tons of photos that you have no use for.. again, don't trash that photo..
Here I will show you how to make a digital abstract background, more or less like that of some famous paintings.
Digital Abstract Art