h4n5Got it.
You have to modify
parseMessage() function for that, not
userInfo_user_custom1(). Before return statement of that function, paste:
/* Default avatar for guests */
if($GLOBALS['poster_id']==0) $GLOBALS['userInfo_user_custom1'][0]='path_to_your_default_avatar';
If the function is not present amongst other add-ons, it may look that way altogether:
function parseMessage(){
/* Default avatar for guests */
if($GLOBALS['poster_id']==0) $GLOBALS['userInfo_user_custom1'][0]='path_to_your_default_avatar';
return;
}