After I upgraded my iPhone to iOS 7, new wallpapers were in order. I wanted something simple and relatively flat to fit in with the new OS design. I went through expected searches on Dribbble and other design heavy areas, but in the end my favorite source was an old standby: Simple Desktops.
Mind you, most of the downloads are more geared for traditional desktop displays, not iPhones. But with a few minutes in Photoshop (usually with me first centering the logo and then cropping it with the proper pixel dimension) I was good to go. Highly recommended.