All users have same Avatar from script

[Bilderback]

I have written a php script which pulls the avatar from the XBox Live Gamercard and want it to display as the default member avatar.
I have disabled avatars in the forum and wish to display one url for all members.
example:
/avatar.php?tag=$userinfo[field5]
The above line gets the members avatar dependant on their Gamertag (field5)
else it shows the default XBox Live question mark image.
How can I edit vbulletin to display the $avatar as the url?
I am willing to edit core files if I have to...
I have tried default avatar plugins and they do work, but avatars have to be enabled which in turn, allows members to change the avatar.
Being an XBox related forum, I would like to set the avatar with no user option to change.

[Dismounted]

Change the URL of the avatar used in the templates to your own.

[Bilderback]

Thanks, I have that within the postbit.
I was looking for a more global solution to span the avatar across different
content areas of the board such as vbadvanced, Drupal, other CMS or products
that may call for the forum avatar.
The template variations may be the best route.