Tattooing is a prison tradition much like making toilet wine and sharpening a toothbrush into a shiv, and most prisoners are proud of their prison ink despite what they may tell their parole officer. However, if you meet a guy who "collects" prison tattoos you’d better ask for another cell, before he decides to shake you down for a contribution!
Here is a collection of preserved tattooed skin patches to satisfy your morbid curiosity, courtesy of the Jagiellon University in Krakow, Poland, photographed by Katarzyna Mirczak. They've been collecting the "skin samples" for decades from dead prisoners, under the guise of cracking some prison tattoo code relating to organized crime, but in reality it's really more of a collection of post mortem curiosities.
And it should go without saying that many of these crudely drawn tattoos have crude subject matter that's NSFW.