If you can't tell, it's a younger, ponyfied Discord dancing with Pinkie Pie. xD
I tried really hard to get Pinkie's facial expression right, and I couldn't do it.

I'm sorry. I fail.

Update: yay! I fixed her face!

Update: So anyways, my theory about this picture is that for some reason (that I haven't figured out yet) Pinkie Pie goes back in time to when Discord first began his tyranny over Equestria (which, for all we know could have started thousands of years ago, hence his younger appearance in this picture) and his younger self sees her and falls in love with her. Then, not knowing that she would recognize him due to knowing him in the future, disguises himself as a stallion with the intention of wooing her. She is quite confused at first, but eventually she realizes that he doesn't know she recognizes him, even in pony form, as the spirit of chaos and current ruler of Equestria. She then understands his intentions towards her. How does she react? I do not know. What do you think?
Well that's my idea. What is your theory of the story behind this picture?