Actually, my hack provides for that! Just edit e.g.
<pre> $theme{'icon_expanded_width'} = 21;
$theme{'icon_expanded_height'} = 17;</pre>

to reflect the dimensions of your expanded.gif - that's all, and this is supposed to go in the w3tthemes.pm - if you upgrade, just copy and paste that section if there are any changes to that file.