Sc uses small_default format to display images in its interface. You can modifiy this format if you like.
First of all, find the format names and size in your PrestaShop backoffice > Preferences > Images or Appearance for Ps 1.7
Then in Sc > Tools > Settings > Catalog > Interface > Size of images in grid: modify the value 'small_default' with the format you'd like to use
Note that the size of images in Sc interface is limited to 80x80, so if you choose a format that is set to 250x250 for example, images will use 80x80 maximum in Sc interface - to avoid latency in loading time.