I’ve never used WP eCommerce, though it might be in my future. ??
Ok most eCommerce have a setting for the images. Usually thumbnails, single product or the like. It depends on what you set these at on how the image will look.
From a lot of wasted time, I would suggest playing with the settings to the sizes you want: set the size/save, look at the product page. If not right delete the image and upload again after you’ve changed / saved the settings. Now this was with WooCommerce that I did this, cuz every change in settings did not change the image.
Yes you can use a plug in to regenerate the thumbnails, but I just did it the way stated above.
I also would suggest using FireBug which let’s you inspect aspects of your site, change code to see first hand what those changes look like and assist with figuring out the css code to use. Remember it’s wise to use either the Themes custom.css (if it’s included) or create a childtheme.css