i made a little change on product_thumb.php in order to save the rescaled picture after the first request.
for example if the picture is named: test.jpg at the first request of a thumbnailed version of that picture it will create the test.jpg.thumb.jpg (i like dots ! ^_^) on any subsequent requests product_thumb.php won't rescale it again but it will simply send the already scaled file.
if you change the original picture you need to delete manually the thumbnailed version.
http://www.oscommerce.com/community/contributions,2226
hope to help.
comments welcome.