It's mostly likely a demonstration to the ex that she still cares about him and wants him back, that she can plausibly deny. OP is probably being used to make him jealous, while still leaving obvious hints that she's open for reconciliation. If she's got public pics still available of them hooking up OP is #2 and she's hoping to hop back on top of #1.

I guarantee she's still FB friends with him so he can see what's going on in her mind. Please verify that /u/ppuummppuumm. And when you reveal it to be true, the excuse of "we're still friends" is bullshit, because she wouldn't have pics of them hooking up together publicly available if they were "just friends".

The sad part is, the ex probably doesn't care (because he would have gotten back together with her by now if he did) and the only two effected here are op and his "gf" who's still hung up on her ex.

I'm sorry for your situation, op.

I hope I'm wrong. And if I am, just tell her you don't like that she has intimate pics of her ex on FB and she'll take them down without question. If not, you're in a bad spot.