Sorry to hear that OP. Something similar happened to me (minus the pic), but I found out the same way. I confronted her about it and she appologised, but never said she would stop seeing/speaking to the guy. the relationship, or what was left of it kept going for about 3 months but during that time I was a paranoid wreck. I'm hindsight, I should have ended it then. In the end she packed her bags and left on Valentine's Day. I believe she is still with the guy now.
I'm in the UK, and we were both on a lease for our flat. When she moved out, she ended up still giving me her share of the rent. I imagine if her name wasn't on the contract it would have been a different story. But if you or her move out, then that could be a way to see the lease through until the end/give your months notice.