function disable_comment_avatar($avatar, $id_or_email, $size, $default, $alt)
{
    global $in_comment_loop;

    if(isset($in_comment_loop))
    {
        if($in_comment_loop == true)
        {
            return "<img alt='{$alt}' src='' style='display:none;' class='avatar avatar-{$size} photo' height='{$size}' width='{$size}' />";
        }
        else
        {
            return $avatar;
        }
    }
    else
    {
        return $avatar;
    }
}

add_filter("get_avatar" , "disable_comment_avatar" , 1, 5);