In order to have a custom avatar, you need to have x number of posts. I believe it's 30, but I could be wrong. There's some other surprises along the way you'll encounter as you continue to make more posts. For example, the reputation system is quite neat. If you see another post you like, you can give that person some rep points, and over time you'll gain rep points and thus have more to give. There's all kinds of other surprises, I won't spoiler them for you. Have fun!