I think Matt (not me, admin Matt) chooses them. I haven't noticed an update for a while, but it's pretty easy to put your own up.
First, try looking in the DC Avatar thread
here. There are 1500 posts, but dig through there and you're bound to find a good Titans avatar.
(edit--I did a quick search, and here are posts with Titans in them. Also, here are posts with Beast Boy avatars in them.) Save the one you want to your desktop. (right click on the picture and 'Save Picture As'.
Then go to your User Control Panel. You should see a link that says 'User CP' at the top of this page right above the grey 'Post Reply' button. After you click the CP link, you'll see a list of edit links on the left. Use the one called 'Edit Avatar'.
Scroll to the bottom of the page (don't panic, we're almost there!). There are two options. You want Option 2. Click the browse button, then find and select the avatar you saved to your desktop (or wherever you saved it to). Click 'Save Changes' and your avatar should change for you.
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