if ( !function_exists( 'get_avatar' ) ) :
/**
* Retrieve the avatar for a user who provided a user ID or email address.
*
* @since 2.5
* @param int|string|object $id_or_email A user ID, email address, or comment object
* @param int $size Size of the avatar image
* @param string $default URL to a default image to use if no avatar is available
* @param string $alt Alternate text to use in image tag. Defaults to blank
* @return string tag for the user's avatar
*/
function get_avatar( $id_or_email, $size = '96', $default = '', $alt = false ) {
if ( ! get_option('show_avatars') )
return false;
if ( false === $alt)
$safe_alt = '';
else
$safe_alt = esc_attr( $alt );
if ( !is_numeric($size) )
$size = '96';
$default = includes_url('images/blank.gif');
$avatar = "";
return apply_filters('get_avatar', $avatar, $id_or_email, $size, $default, $alt);
}
endif;

即使用该函数，仅可能返回一个默认头像（位于wp-includes/images/blank.gif内），再配合simple local avatars或Add Local Avatar插件，就实现了预期的效果。