Add the follows scripts into function.php
[php]
add_filter( ‘avatar_defaults’, ‘newgravatar’ );
function newgravatar ($avatar_defaults) {
$myavatar = get_bloginfo(‘template_directory’) . ‘/images/deafultAvert.png’;
$avatar_defaults[$myavatar] = “newImg”;
return $avatar_defaults;
[/php]
Save the js file and login in admin panel , choose ->setting ->discuss -> choose your image