Well, I take most of my backgrounds from render sites across the net, deviantArt and my self-made images (be it photographs or digital stuff). It usually takes me about 20 minutes to half an hour to find the background with the right 'feel'.
And sometimes I stumble upon a graphic that I can use immediately 'cause I just know what to use it for.
Just keep on doing those sigs, you automatically learn with more practice. Oh, and no harsh pastel-stripe render-filter backgrounds please!