[MarketPress eCommerce] Problem with Marketpress images in list view

I'm building a Marketpress store at http://theolyn.com/soulschool/books/ and I'm having trouble getting the products list view to look right. I've looked around the documentation and the forums here, but haven't found anything that helps. I hope you can help!

I've set the product thumbnails to display at 100px x 151px and I note that Marketpress has created images that size from the main product images (which are 350px x 530px). It's correctly using these images for the product list view presentation, however the graphics themselves have been upscaled on screen, meaning they are rather blurry. 100x151 graphics are displaying at around 4x their proper size. I can't find what is telling Marketpress to do this — can you point me in the right direction, please?

I'm a book designer who has been obliged to work on web development, rather than choosing this work, so please don't be too technical in your reply! (I should also mention that the best thing about Marketpress is its simplicity of use — something non-developers appreciate! Thank you for making this amazing plugin!)

Best wishes,

Will