If your thumbnails are all the right size to start with (100x150) then you could change your skin1/product_thumbnail.tpl to this
Code:
{* $Id: product_thumbnail.tpl,v 1.4 2002/09/05 12:43:21 zorg Exp $ *}
{if $config.General.show_thumbnails eq "Y"}[img]../image.php?productid={$productid}[/img]{/if}
In other word just remove the part that sets the size