This is how Derpy got her cutie mark, she originally had a cutie mark of a letter, but that changed after this dreaded night.
Derpy was just going out as always, looking at the stars at the edge of the Everfree Forest. Suddenly she saw a beam of light coming down from the moon wrapping around her. She felt a burning sensation on her flank, where her cutie mark was. It was lighting up and changing form right before here eyes. She was very scared, she didn't know what to do. They strange light that emanated from her cutie mark seem to calm down, but it was hard to tell what it was. The light fled as Derpy slowly tried to get a grasp of what happened. The cuitie mark... it was, bubbles? Or so she thought! But it was really stars, given to her by Nightmare Moon herself. She had split her power for both the moon and the stars and gave Derpy control over the stars without Derpy knowing. Because she wasn't as trapped as Celestia first thought. She was slowly planning her escape, laying down the pieces for her upcoming empire. Derpy was simply a piece in her diabolical chess game. She has anticipated that Celestia would train up someone in the hope that she would have the magic inside of her to defeat Nightmare Moon, but little did Celestia know that she had already planted a seed among her people, to someday when her plan comes into life, she will use it against her to regain Equestria and banish Celestia for what she did to her.
That was a lot longer than I thought, but you get the general idea behind the image.
As for the image itself, I tried a lot of different backgrounds. First some simple graphics, but that didn't quite work out. So I resulted to back in some of my old style Photoshop-ing. It was an attempt to be a bit more artistic as I'm not to good at drawing, more manipulation. But it was interesting, and I like the backstory I came up with ^^And fanfics writes, here is your story. Personally I wouldn't consider myself much of an Author. So I'll leave it for you if someone wants