Based on what the picture looks like, I have no reason to doubt it's authenticity. However, some years ago there was a Dutch documentary shown over here in the Netherlands where they interviewed the man who actually did Pac's autopsy. He couldn't really remember much of the autopsy itself because it wasn't really a big deal to him, seeing that he's done hundreds of them and he didn't even know who Pac was. But he did say that he couldn't see anything strange about the autopsy photo and he thought the photo was probably legit, although he couldn't say for sure.
But then, mysteriously enough, when they are finishing up the interview and are preparing to leave, the pathologist's assistant walks in and I think he did remember it or something, so he takes a look at the photo and immediately noticed something about the photo that didn't seem right to him. I don't really remember what it was, I think it had to do with what Pac was lieing on, or the attributes in the picture or something.. or the color or whatever. It's been a long time since I've seen it.
Anyway, basically they couldn't say for sure whether it was real or not and said it could be either way. But what would you expect.. I mean, it's Pac.. it's bound to be a mystery..a myth...
Edit: Nevermind. I found a site with the documentary on it and I watched the bit with the coroner again and he says what I just posted, only the assistant part isn't in it. So, either I dreamed it, or it was in another documentary where I saw that bit. Anyone know perhaps? Anyway, here is a link to the documentary if you're interested. The woman in the documentary is absolutely ignorant when it comes to Pac ("Toopack") and she's just horrible with her background knowledge, but she did somehow manage to interview some key figures. Afeni,Mutulu, Kevin Manning (police officer), the coroner, Cathy Scott and a couple of others. Anyway, the autopsy part starts at about 20:00 minutes.
http://player.omroep.nl/?aflID=3220119