If it would be alright, I'd also like to use this as cover art for a story. It fits perfectly, and I'll also give credit where credit is due. May I use it? ^_^ I'd also send you a link to the story, so you can see what your work is being used for.
"Luna...my dear sister..." She whispered. "I was protecting my kingdom....I hope you understand..." Celestia looked up at the moon. "Was it worth it...? To banish my sister....?" She asked herself. "I know I did the right thing....but it still hurts...."
1000 years is one thing, yes. However... at the time that this drawing presumably is taking place, the Princess honestly thought it'd be forever. Remember, the only reason Nightmare Moon/Princess Luna ever returned is that the stars took pity on her. It had nothing to do with a relative impermanence of the original spell.