Dynamic resizing creating weird and bad images on my product pages

Hello everybody, I'm new to Squarespace and really floundering - would anyone have any ideas about how to handle dynamic resizing of my images? 

I assumed the aspect ratio of my images would be considered paramount and that the whole product image I created, even if it appeared smaller, would appear normally. I've attached a screenshot of the original product image and a screenshot of how it's appearing on my site. 

I sell postcards, so some of them are horizontal and some are vertical. The vertical ones look OK (at least on my desktop). (Screenshot also attached.) I assume there must be standard image dimensions somewhere so I can resize my images, but I can't find them anywhere - would anyone know? This is doing my head in. 

13 minutes ago, farawayplacespaper said:

I'm new to Squarespace and really floundering - would anyone have any ideas about how to handle dynamic resizing of my images? 

Please take a look at my guide below to see if this helps:

Product image aspect ratios on Squarespace 7.1

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To anyone else hung up on this idiotic topic: 

I addressed this by screenshotting the image as it was appearing in its dynamically resized state, then figuring out the aspect ratio (actually via chatgpt because I was feeling extremely lazy). It was 4:5. I've now resized all images accordingly and it looks as good as horizontal products can look in a vertical product image space.