As Blazing says - it seems (an inexact science ?)you need to have accrued a certain amount of posts &/or reputation to be able to use a custom avataar.
You also need to have an avataar that meets the size requirements (in the 60-70's kb I can't recall exactly)
Go to your User CP every now & then I attempt to change it - that's again not science but the way I fell into knowing/doing.
Cheers
rayC
I don't think it's a post count thing. If I recall correctly, I put my avatar in at about 30 posts and my custom title about 60. That's when I first TRIED to do it too, so for all I know I could have done it sooner.