I couldn't find a mod to suit your need. I think they made a simple template modification. My thought would be to design an avatar background as they did.
http://www.gamingforce.org/forums/images/bb_avatar_back.gif
Then go to the SHOWTHREAD template and look for the code that deals with displaying avatars. You can write up a simple HTML code.
<TABLE width="131" height="176" cellpadding="0" cellspacing="0" background="http://www.gamingforce.org/forums/images/bb_avatar_back.gif">
<TBODY>
<TR>
<TD align="center">Avatar Display Code here</TD>
</TR>
</TBODY>
</TABLE>
Basically this is a clear table (no borders, no padding) set to the size of your avatar border background image size. In the table would be the variable to show the user's custom avatar. The avatar bg image would be set as the table background. Within the table would be your avatar display variable centred both horizontally and vertically. I guess once you set the image avatar margins to the same maximum size of the inner border of the avatar border background image, it should display properly. Also set it within the conditional tags so it won't show up if the user does not have an avatar.
This is only a framework of what could be done so experiment a bit. Hopefully someone else knows the avatar display variable. Sorry i can't help you more but i don't know the avatar display variable name.