My guess about the Celestia picture is that it turns out that Twilight is actually Celestia's long lost sister who was kept alive via reincarnation. When Twilight found out that she had been lied to, the memories of her past lives spilled back into her mind, slowly drowning her in insanity.
Troubled about her deteriorating condition, Twilight goes to Zecora's for a cure only to destabilize and turn back into her true form as an Alicorn. Hellbent on vengeance, she obliterates ponyville and goes back to canterlot to duke it out with Celestia and Luna.
I think that's the most logical assumption. I can't think of what else it would be...
