If you have a picture of an attractive person on your avatar, you will get a lot of positive attention from the opposite sex (or same sex, if that's your thing). You'll be noticed. I don't see a thing wrong with that. If someone has a picture of a rabbit on their avatar, I know they're not a cute fuzzy rodent. If someone puts a picture of Kiera Knightley on their avatar, I'm going to be pretty sure it's not Miss Knightley blowing some time between scenes.
If you develop an actual relationship with someone that will go offline, then of course you must show them what you really look like. Otherwise, pretty people have easier lives. It's just the way of the world.
Edit - I did want to say that I'm referring to first impressions, only. An attractive avatar is going to attract more initial attention. Call me naive but I believe *most* people see the inner person once they get to know you.