Sweetie's cutie mark is explained in my fanfic, 'Scion of Chaos', and the meaning behind Scootaloo's cutie mark is something that Scootaloo is still struggling to understand on her own in 'A Heart of Change'. If you feel like reading an children's adventure, you might want to give 'Scion of Chaos' a read. Many of my readers have quite enjoyed the story. But I can also understand if you wouldn't want to read a novel. It should suffice to say that there are sound reasons and context for the cutie marks to be the way they are.
I do have to agree that it's a lovely picture. I'm quite grateful that Tails was willing to commission this for my story as a birthday/Christmas present. It was a very flattering gesture.
I just love this fic so much, just finished reading up to the current chapter and am so eager to keep reading, I was especially stunned to discover that it had a chapter for The Sweetie Chronicles: Fragments, which I'd forgotten to monitor lately and had to spend time catching up on before I could continue A Heart of Change.
It's those enormous and amazing collaborations that remind me of just how awesome the MLP community is