I would hazard a complete guess to say it's either 1) Shep and that tattoo artist borrowed from the same source, 2) That tattoo artists modified Shep's work, or 3) Shep or SNO designed that for that artist.
Tattoo artists have big books filled with royalty free images that they can pull from. I know in the past Shepard has admitted to pulling images from these tattoo books.
So I'd say there's a good chance they both took the original image from the same (or simliar) royalty free image books.