Well my profile picture is just my OC and I made it to have an unique avatar. By the way I'm not a good artist, so her look is kinda a result of me trying to make a pony of happiness. (I plan to recreate her someday ~ my sister is a good artist, so perhaps I'll ask her for help...?)
Anyway why rainbow in the background? To represent happiness, energy and colorful approach to life, that she has. =3
So... generally it's a very simple avatar, nothing special.