I think you have a good idea for a tattoo, and if people think you're vain then they are the stupid ones.  Judgemental people aren't worth worrying over anyway.  Get whatever you want.  Whatever is meaningful to you.
 
I self-harmed for about 9 months, I cut my arms and my legs.  But I don't scar easily and I only have one scar on my thigh.  So when I'm 18 I want to get a tattoo over it that says "The Shadow Proves The Sunshine".  Maybe consider a short quote or just one word, like Hope or Live.  Something that maybe has helped you through your experience with self-harm.  Good luck.