This is going to be part of a welcome package.
If you use one please leave a comment to say which one. More to follow.
You can download one of these to your computer - click to see it fullsize, then right click and select 'save image as', choose where you want to store it on your hard disk, and click 'save'. Or find any pic on the Forums or elsewhere on the net and download it the same way.
[Note: several of these avatars are now in use - 19.02.2013]
Once you've got a suitable pic to use for your avatar stored on your computer, it's quite easy.
1 Put the cursor on your name near right end of top bar, don't click until a dropdown menu appears,
2 then click 'Avatar' on that menu,
3 click 'Choose File',
4 which will bring up your own hard disk directory, where you find and double-click on your chosen pic.
Then it should appear in the Avatar box.
If it doesn't, it's probably because the pic's unsuitable in some technical way, you may need to try one or two others.
If you have any further problems, do contact any staff member -the home page shows which staff are online - click on their avatar, then on 'Start a Conversation' on the ID card that comes up.