Want to show user image on the header of every page

[ltwinnerr]

When a user is logged into my site I want to show their avatar in the header of every page.

I thought it would be a simple matter of retrieving user image from some global variable but after looking at the avatars in the forum, which have src attributes like "image.php?u=1&dateline=1312310668" and did some debugging with netbeans and I can see it is more complicated than that and the avatars are retrieved using the image.php script.

I have inserted a template_hook.user_pic into my header template and I have a plugin that registers the variable -

$template_hook['user_pic'] = xyz

All I am missing is the xyz. So how can I retrieve the image of the currently logged in user so I can replace the xyz with the users image? Cheers.