Hi, and welcome! I asked ManagerJosh on this and we don't have the option to use our own avatars. I forgot to ask why though. Hopefully someone will know why. I'm going to be stuck with my mini-me for a long time if this doesn't change, lol.
I can answer that. To prevent short-term members from filling up our servers with custom avatars we've limited custom avatars to members who have either 900 posts or a Reputation level of 300. Reputation level is given to you by other members if they vote on your post as being helpful or not. If you look to the left of all the posts on the bottom is a little green circle if you're online, grey if you're offline. Next to that is the judicial scale. Put your mouse over it and it says "Add to jupitershana's reputation" Or whoever you're looking at. Click that and it will give you the option to either approve or disapprove of a post and to add comments if you like.