Here is all--all!--derpibooru has to offer you:

Besides those, the Minuette-ventures contrived to get another image from somewhere. But that seems to be all. No wonder there are so few stories when there are so few illustrations! (Third-Cause Fallacy? never heard of it.)

4532586
Sharp Spark commissioned the last picture referred to from Ghost at my behest, shortly before her, uh... passing. :( The next time we needed cover art we had to make due with you, it was terrible! :p

Besides those, the only two other extant pics I could think of were, uh, Strawberry Jelly's avatar

and the cover art of this

(apparently that is actually Clover the Clever and the fic ends in Berrygate, so don't get your hopes up)

At least there's some stuff from the new episode, though! I mean, mostly just random screengrabs, but there's also this!

it might be like the best picture ever

Anyway, I take it from your search for cover art that The Twigate Story (ft. Lemon Hearts) is finally complete and ready for publishing, and you are just working out the last steps. This is clearly the only explanation.

(Are we actually supposed to just hotlink from Derpi like this? Isn't that bad form or something? Also how did you get urls into the images, the tags wouldn't play nicely with each other for me.)

4532655 Ahh pretty pretty pictures~

you are just working out the last steps

It's possible. I'm tempted to use a picture of the GIANT HOURGLASS TWILIGHT KEPT IN HER CANTERLOT QUARTERS, because you really can't ask for a better sign they were meant for each other, but that might not be eyecatching enough as a story image. Idk.

(I don't know what is bad form or not, but hotlinking from Derpi is a lot easier than from some other sites, so I tend to do that.)

I'll be putting up a doodle of my own in a few days, probably, (dependent on me figuring out how to make a gifs work) but I can't promise it'll be good enough to work as cover art. Or even particularly good. Still, more exposure never hurts, right?