Make thumbnail images fit in the box

I’ve got great images for my ebooks which look fine on the actual product page, but on the category listings page they don’t fit in the thumbnail – how can I change it just for these two products that fall into this category (in this case the category is ‘ebooks’). I haveg another product category ‘Christmas cards service’ but I am happy with the look of the product images and category thumbnails. So I just need to change it for the two individual ebooks per the link above.
Please someone help me! I want to launch my ebook today but don’t want to do it until the thumbnails look their best.
I want the entire image to fit into the thumbnail – how can I do this?!!

The page I need help with: [log in to see the link]