I find it funny that people are so instinctively willing to doubt that this is real. They thought the same about Twilicorn without considering the full picture and they were dead wrong. Looks like they didn’t learn their lesson.
Just like they weren’t bright enough to consider the idea of Twilicorn happening at all under any circumstances, they aren’t bright enough to consider that this image doesn’t necessarily mean Celestia’s death. All they do is shout "fake, hasbro wudnt let Celestia die!" Well, thank you for pointing out the obvious… How about changing your perspective on the image just a little, hm?
So basically Twilight somehow goes back in time, NMM beats Celestia, Twilight rages in sadness and banishes NMM to the Moon, Twilight somehow blabs it to Celestia that she time traveled to see what happened then "altered history", Celestia takes credit, knows Twilight will exist in ~1000 years, makes sure she becomes her student.
if the Discord pic is real then it is safe to assume the first time travel screws up history and she has to go back and stop herself.