I did not make any changes to this test install. Yes, images of Argent are 75px but when changing image size of "Thumbnail 1" they get this new size as well. But "Thumbnail 1" is image at shop page, so it's another image. Please check site again, size has changed: http://webmastergroningen.nl/qc/?1,fusce-felis-enim
We do not understand each other. I do want the preview image. Main product image is 250px by default, extra images at product page are always 75px in free version.
In free version you can only change size of Thumbnail 1 > that's the image on page with all products. In ext version you can also change size of Thumbnail 2 > that are the extra images of a product.
If you add more than 1 product image in free version, every image gets the same size as Thumbnail 1. That is not how it should be. The extra images should always be 75px.
You can try for yourself, by installing free version of QuickCart and adding new product images.
I can't help you without access to your shop/server. It works. Look at image with my test. https://www.studiostrona.pl/files/images_250_75_free_v67.png