Welcome to 40konline. Enjoy the Community!
The proper choices for uploading and selecting online avatars are locked for first-time posters (as a security measure) and will become available as you post. Specifically, obtaining less than half a dozen posts will drastically alter the way you can use your profile. This does not mean that you should in any way spam your way to 10 posts or so to get hold of a better profile. Such behavior does not reward itself here.
Also please note that certain boards do not generate postcount, like the Tavern and the Trading Post.