Every now and then I get contacted by people interested in my Sokoban clones, or some aspect of them. Often it is the levels and/or how to parse them, but often it is the graphics.
Since Sokoban is a popular game to program, I figured I could help aspiring game authors out by providing free graphics. So, I've published all Sokoban game skins created by me with a liberal Creative Commons license on GitHub.
Have fun: github.com/borgar/sokoban-skins