I don't know if this sounds harsh or insensitive but..
YOU ARE DIVORCED!
You said the picture was from June. AFTER you were divorced.
And what is it? A picture of your X with OM and other friends. You can "tell" a lot from the photo, but like I said. What does it matter? It's not an A at this point.
The reason I say this is just because how I can envision your "calling her on it" will look.
You, her XH, is spying on her FB page and is now expecting/demanding answers about a photo he saw of his XW with friends.
What does this say? About you? About your life? Your progress?
I TOTALLY understand that it does hurt. I understand wanting answers. But is it worth it? To YOU? FROM her?
BITS
Happiness is not something ready made. It comes from your own actions.