Alter the image used for a user who has not uploaded a custom image yet

Here’s a function you can put in your theme’s functions.php file to force your own default avatar. Update the $myavatar line to point to your image path/name:

// Adds a new default avatar to the list in WordPress admin
add_filter( 'avatar_defaults', 'mytheme_addgravatar' );
function mytheme_addgravatar( $avatar_defaults ) {
    $myavatar = get_bloginfo('stylesheet_directory') . '/images/my-avatar.gif';
    $avatar_defaults[$myavatar] = 'New Default Gravatar';
    return $avatar_defaults;
}

Once you’ve added this function, your new avatar will appear in the list in the WP control panel in the “Settings > Discussion” area. Select it and save. Now if visitors don’t have their own Gravatar your new avatar will appear instead.

(If you’re just looking for how to get your own Gravatar, which will show up on most WordPress blogs/sites in the comments section, go to http://gravatar.com and sign up for a free account.)

Hope this helps – best of luck!