Wordpress Avatar based on username

I am trying to add a wordpress avatar based on the username, so far I have added the following to the functions.php file in my template to change the default avatar and it works well:

if ( !function_exists('fb_addgravatar') ) {
function fb_addgravatar( $avatar_defaults ) {
$myavatar = get_bloginfo('template_directory') . '/images/pending.png';
$avatar_defaults[$myavatar] = 'xboxrox';

return $avatar_defaults;
}

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

Now I can change the $myavatar string to a remote url of an image and that works, however the image I want to use is the avatar icons from xbox.

http://avatar.xboxlive.com/avatar/{$user_login}/avatarpic-l.png​

Where the user_login = the members gamertag

This is where my problem lies....
Wordpress adds a '&s=64' or a '&s=50' after the url for the image.

http://avatar.xboxlive.com/avatar/ElusiveLeader/avatarpic-l.png for example works perfectly but
http://avatar.xboxlive.com/avatar/ElusiveLeader/avatarpic-l.png&s=64 returns an error.
Is there a way of removing the '&s=64' or the '&s=50' resize attributes to call the image?

Please help, I really would like to set the users avatar in wordpress automatically to match the live image held at http://avatar.xboxlive.com/avatar/{gamertag}/avatarpic-l.png

 

I have fixed the above error by installing add-local-avatar plugin

I have in my functions.php:

if ( !function_exists('fb_addgravatar') ) {
function fb_addgravatar( $avatar_defaults ) {

$myavatar = "http://avatar.xboxlive.com/avatar/{$username}/avatarpic-l.png";
$avatar_defaults[$myavatar] = 'xboxrox';

return $avatar_defaults;
}

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

But the image returns:

http://avatar.xboxlive.com/avatar//avatarpic-l.png

How do I insert the users username to the above url???