What's a "sub-forum?"
This sounds like one of chess.coms many random and temporary glitches. If it doesn't correct itself in a few days contact the staff directly via Help & Support at the bottom of the page. I'm not aware of any settings to turn your avatar on/off or to only display it in certain places.
Common problem & simple answer - since you've been a member for less than a day here, it may take up to 24 hours for your avatar to start showing properly.
Just be patient 
If this problem persists, best to report it....
http://support.chess.com/Tickets/Submit
OK thanks for the fast reply. Hopefully things will be normal tomorrow or the day after that. Sub-forums are just those you see when you click on any of the main forums, like the stuck topic "Chess rating system" is a subforum while "General Chess Discussion" is a (main) forum.
(Wow, whatever happened to the font...)
Strictly speaking what you call a 'sub-forum' is known as a 'thread' on this site! Then we'll know what you mean 
As I browsed through the forums, I found out that my avatar wasn't showing, even though I was the latest poster, but it DID show when I reached the sub-forums... Why is that and is that just a bug? Or are there any other settings that I'm not aware of? Thanks in advance.