You can find the polaroid style on this page: https://demos.wpshowposts.com/cards/
The CSS I mentioned above is the cause of the issue. The min-width: 560px is what makes me think it’s custom CSS, as we don’t use that media query in the plugin.
The only reason the image quality would suffer is if the image is smaller than the size it’s displaying at. What if you use image resizing with a width/height that’s larger than what it displays at on the page?