These shirts are 100% cotton Hanes Beefy T's, with a small TR logo on the front right chest and the image at the right on the back. If you'd like to see a larger version of the image, lookie here.
Some folks have asked how well this image will translate to a shirt, and here's my best answer: These shirts are "printed" in full color at 100 dpi (color resolution), so it ought to look great. The source image from my digital camera was high-res enough we were able to meet CafePress's resolution requirements exactly. I don't have a Jr. Damage shirt yet, but the other shirts don't have any jaggies on them, and they're all created from raster source images, so fear not.
Mad, howling props to Dissonance for putting this whole shirt thing together for us.