// Add a default avatar to Settings > Discussion
if ( !function_exists('fb_addgravatar') ) {
	function fb_addgravatar( $avatar_defaults ) {
		$myavatar = get_bloginfo('template_directory') . '/images/avatar.jpg';
		$avatar_defaults[$myavatar] = 'Doc4';

		return $avatar_defaults;
	}

	add_filter( 'avatar_defaults', 'fb_addgravatar' );
}

http://wordpress.org/support/topic/custom-default-avatar

Add the following to your active theme's functions.php file then upload an image named "avatar.jpg" to your active theme's image folder. The image can be named anything you would like but be sure to change it in the code below as well.

The last step is to visit: Settings > Discussion to select the new avatar as the default.

Wordpress - Custom Default Avatar

wordpress

PHP