Let me give an attempt at exegesis:
The mill and the smoke stacks obviously imply a sort of industrial revolution. The rest of the picture implies that something was lost when technology progressed.
The tree house in the picture implies that the treehouse no longer exists (that was oddly prescient). The presence of pipe (a steam pipe?), square windows, and what looks like a river (I'm not sure) out the window give further evidence that this takes place in a different location than the library in the show.
The windows, pipes, and columns are covered in muck. I'm guessing spike is gone. The poster is decayed and old. Clearly this building has been around for a while, implying this is in the relatively distant future, and the mills did not spring up over night.
Rainbow Dash is holding goggles, so evidently she has a job that relates to flight, most probably as an airship pilot or some sort of profession related to the evident technological progress.
The words "Final Show" on the Wonderbolts poster is the most hauntingly tragic detail in a picture full of hauntingly tragic details. This just further emphasizes the running theme of "progress came and took its toll."
Rainbow Dash and Twilight are both sad, Rainbow Dash more so. This doesn't seem to be the sadness of a longstanding feeling of detachment and loss (which the other imagery in this picture invokes), but from a recent event or revelation. My guess is that Rainbow Dash has stopped by, wearing her work goggles, to see Twilight for the first time in years. They are both lamenting their lost friendship, a friendship lost because technological change has caused them to pursue vocation in different cities. Rainbow Dash's side glance indicates awkwardness, shame, or guilt, and Twilight's view over the railing indicates reminiscence.
Twilight's tiara is on the shelf. It's presence in the open (unlike in the show, where it was locked away) implies that it has recently been used or is expected to be used, but its location on the shelf indicates that Twilight doesn't have much love for it or what it represents any more.
Fluttershy's cutie mark is on a book. Did she write a book? I’m not sure what to make of this.
By far the most bizarre aspect of the whole picture is the fact that the ponies don't look a day older than they do in the show. Every detail, from the lighting to the dirt on windows works towards building a sense of loss. Why not invoke the image of lost youth as well? It looks like decades have past, but they are still young adults. Are they immortal? This is just a small oversight, I guess.
Not a thousand words, but definitely a respectable number. I guess a person could say a whole lot more about Rainbow Dash’s expression or that haunting poster, which alone is enough to put me into a miniature depression, but this was my attempt. I hope I did it justice.