Comment avatar for people who don't have one ?

Some peoples ask me how to add an image for people who don't enable their avatar in comments . Yes, I missed this function when making template . In my templates, you see nothing in avatar frame if people who comment don't enable their avatar . Why don't we add an image in avatar frame for them instead of using blank ?  Like this or like this (^^make it look like Gravatar)the trick here is very