Image resize trouble

I have an online store and a couple of product images look a bit dodgy. The store has one image per product and scales it down in the browser to create thumbnails. My problem here is two of them, when it scales down these line drawings a lot of the lines dissapear. At first i thought it was becuse they were gifs and the rest were jpgs, so I changed one but it made no difference. I then realised the problem images were a lot crisper than the ones that work so i tried thickening the lines as I thought maybe it was trying to scale a line 1px thick to be smaller. But that has made little difference too. Here are the images, http://www.cotswood-doors.co.uk/catalog/external-doors-handcrafted-c-1_7_11.html the problem ones are ‘cottage 3 light vertical’ and ‘cottage diamond light’. The first is the new thicker jpg and the second is still a gif. Thanks for any advice you can give, this is driving me mad!