You can find my own t-shirt designs right here. You can download the sources and the imagefiles for free.
Larry Ewing's original Tux drawing, created with The GIMP in 1996, is used in this design. (See more details on the Tux mascot here: https://en.wikipedia.org/wiki/Tux_%28mascot%29)
You're allowed to make your own prints and/or modify the design. I'd appreciate, if you'd write me a line, but you're not required to do so. It's licensed under CC-0, so do whatever you want!