Right now I also don't see how this picture means that Pablo is evil.
I mean if he really wanted to blow Diesel's cover why would he do it that way?
It's just a reflection of Diesel in a mirror. There's no guarantee that the right people see this picture or even notice Diesel in it unless those people know that they have to check Pablo's account. But even then there's still the problem that the picture can been seen by everyone, including Ein and Diesel. Which means they could notice it and have the chance to flee before the bad guys show up.
It would be a lot easier and more efficient to simply show the bad guys a picture of Diesel instead of posting a reflection on social media.
To me, it seems Pablo didn't give much thought to the picture (and probably didn't even notice the reflection, because his attention was on something else).
I really don't think pablo meant anything bad about the pic (i really like pablo i hope he's not evil (〜￣△￣)〜) ..but it's true this might bring drama for pretty boy diesel