These were hard to see sometimes and some artists have tried not to use them by covering the arms up. However this is Rocaforts' design sheet for said tattoos which was released online:
There has been no explanation for Roy choosing these although some of it can be figured out like the skull with the bow and arrows sticking out of it's mouth. Perhaps they also serve as a reminder not to get lost in addiction again given the scorpion and "Poison." On the left arm is "pesadilla" (nightmare.) Although I wasn't able to read it there was one panel that showed another tattoo lower on his arm which was translated by another reader as "crossbow." (It's been forgotten under other artists.) So needless to say archery and his past demons does prey on his mind.