On fimfiction.net, a story's author can link from the story's cover image to the cover image's source.
I make some of my cover images by cropping (selecting part of) a frame from an episode, movie, or related canon MLP video. Because there's no direct source for my image online, this blog post might be the closest thing to a "source" I can link!

That's how I got cover images for stories like the examples below.
After a nasty breakup with Sunset Shimmer, a Twilight Sparkle discovers a way to travel to parallel universes. Millions of Sunset Shimmers who've never met her. It was a perfect plan...until it wasn't.
Mockingbirb ·
7k words
· 53
4 ·
1.9k views
Cadance wonders what Flurry Heart is the alicorn princess OF. Twilight Sparkle turns perfectionist (Ha! I'll bet you never saw THAT one coming!) and tries to figure out who's more inadequate, Flurry Heart or her mother?
Mockingbirb ·
1.6k words
· 58
1 ·
1.5k views
Fluttershy's friends might call the gentle pegasus sensitive. But they don't know the half of it. What is she always so afraid of, anyway? And why is SHE the one pony who can go toe-to-toe with the Avatar of Chaos himself?
Mockingbirb ·
2.6k words
· 296
3 ·
3.5k views
"Traditional changeling family values are different," King Thorax remarked. When hundreds of angry changelings attacked Twilight after dinner, she wished she'd asked for a better explanation.
Mockingbirb ·
1.9k words
· 63
3 ·
1.4k views
Late one night, Twilight finds out about a secret shipper. A shipper with a deliciously naughty voice and cute little fangs, who doesn't want to take no for an answer.
Mockingbirb ·
1.7k words
· 58
2 ·
1.5k views