They are called avatars and are just images that each member picks for what ever reason. Of course, I can't find the thread that explains when and how you get them. Anyone can have them providing they meet some requirements (usually a minimum post count and
a minimum lenght of time as a member). I don't think points have anything to do with them though and I'm pretty sure the only way to get a bigger one (you'll notice some are small and some are big) is to become a paying member.
ETA: Found it...
You can find details about user groups and what you can do with them here
* 1 post and 1 day in membership
* 10 stored PMs* 5 avatars to choose from
* 700 posts and 120 days in membership
* 40 stored PMs* 39 additional avatars to choose from
* Ability to upload custom avatar
* 80 Kb in uploaded attachments
* Posting access to IMO, if over 18.
SO, you should be able to now. Just go to your "User CP" and the second to last option under "Setting & Options" should be "Edit Avatar". There you'll find the 5 avatars available to you now.