After reading this I went back and checked out prior episodes as well and there is no question about it, in EVERY other shot I saw, Tom's tattoo was on his right shoulder. The only shot where it was on his left was when Tom and Lex were going down to the water hole for the first time.
I think there are 3 possible reasons for the tattoo to be on the wrong shoulder:
1) We are not shown Tom, we are shown a stand in, and the make-up people applied a temporary tattoo to the wrong shoulder.
2) The picture of Tom is shown in it's mirror form. ie. when they edited the hours and hours of film that gets cut down to 40 minutes of show, whoever did the editing, intended or not, flipped that clip of Tom.
3)The picture of Tom is the only clip not in it's mirror form. ie. Everything we've seen up until that clip was in it's mirror form (to disorient spoilers trying to match things up with maps). The reason that specific clip isn't in it's mirror form is because Lex and Lindsey are talking about how to get to the water hole. Lex starts to go right, when Lindsey tells him to go left. Lex and Tom then start to head to the left. If this was in the "mirror form" then Lex and Tom would appear to head off to the right after Lindsey said to head left.
These are the only possibilities that come to mind.