A gravatar problem with my theme- probably simple

OK, I am new to this whole WP thing and feel rather guilty asking for help and not giving any but to be frank- I am utterly useless with this.

Background:
I have created a theme (modified from Publicizer) and have made a small table at the top in part of which I would like to display the current users gravatar (as a visual representation of who’s logged in)- this is going in my header.php file.

Problem:
The gravatar displays the little greyed out default man (when logged in) when I am using my current attempt at the code:

<?php
   echo get_avatar( $id_or_email, $size = '100', $default = '' );  ?>

The comments area still displays gravatars fine so I suspect it may be something to do with “id_or_email” but I am inexperienced don’t know what to try.

Many thanks in advance to anybody who can help and I’m sorry if this is in the wrong forum.