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 simple .
Just find this code :


and replace it with


with this code ,if people don't enable their profile images (or their Blogger avatar in comment ) it will show image <img src="http://img844.imageshack.us/img844/3447/unknownuser.jpg"/>
you can replace image which I marked in bold to your own