well, you dont want the echo… the sp_UserAvatar() function (as per the codex page) already outputs the data unless you set the echo=0 argument in which case it would be returned…
also, since you are not passing anything to the function, its assuming it will use the avatar of the currently logged in user… is that what you want?
do you have a link to this example? would like to see if any output is generated just to make sure the function is really getting called (ie not missing an include file)…
if you want it for a specific user vs current logged in user, you would do something like
<?php
sp_forum_api_support();
sp_UserAvatar('context=user', $user);
?>
where $user is the user id or user email address that you want the avatar for…