How do you upload your avatar?

As far as I know you go to 'manage photos' and select one as your avatar by clicking 'set as avatar' next to the image.  You just can't use one with an 18+ rating which is why you might be having problems.

It seems that your collections of pictures are Not Avatar Ready. "M" cannot be shown as a Avatar. Perhaps a pretty picture of a kitten or puppy.

Un "M" one of your images. Then choose it for your avatar. In the next step you'll have to crop it so it's avatar friendly (No twigs or bits showing). After it's uploaded as your "Avatar friendly PG version" you'll want to go back and make it an M again.