This simple photo of a tree trunk makes a satisfying computer background. It has a mix of different surfaces at several angles and varied lighting. You can see how the bark split as the tree grew. The light coming from the left emphasizes the texture.
Photography notes:
The sides of this original image are soft because the round tree put them just outside the of the area in focus. They have been cropped out in the presented photograph. The bark was lit by sunlight filtering through the leaves of nearby trees.