It appears to be a grayscale version of an image likely called Patterns in Nature. The largest version of it I could find is located here, and I've attached the terrible reconstruction I performed to do the image search, leading me to the minuscule information I could find on it.
Perhaps you'd have better luck posting in the UI Customization sub-forum, so I've requested that this thread be moved there where photo wizards Tiggger and Firedept might be able to work some magic.
I wasn't able to find it anywhere; the best I was able to do was edit the image itself into what's attached. It's one of those mist-rising-from-mountain-valleys pictures that looks a tiny bit like this. Hopefully, Tiggger will have better luck.