When to Respond Negative, Neutral, or Positive

A girl I met at the club, and have known for some months now sent me a selfie out of the blue. It's mostly a normal selfie. Aside from fitting in her cleavage with a slight duck face. We had a good vibe or a while. Until she moved into the city and started bar tending. Conversation was mostly initiated by me after her interest started to go down. Why she's messaging me out of nowhere I don't know. Things only fell by the wayside as they sometimes do. Here's the thing. There are several principles that are conflicting: Matching someone's energy. Positive Reinforcement Push/Pull Etc. Here, I'm not sure how to proceed. Responding in an underwhelming manner might not be the right "Push" at this time. Rewarding her doesn't seem right either. So, maybe something neutral such as: "You again? What's up troublemaker." Could be appropriate. Thoughts?