Voyager

That looks normal to me, all preview pictures have the same aspect ratio.

I don't use this app, but previews should have some mechanism for limiting the image file size, and it is entirely reasonable IMO that one possible strategy for automatically limiting file size for the preview image might be to clip a chunk out of the middle of the original image.

I find this annoying as well. I wish images were only cut off in the case of ridiculous dimensions.

I don’t know what’s correct; maybe I’ve just gotten used to this?

But, specifically, my experience is with external links to images the images appear full size while with links to websites the hero image (I don’t know if that’s the correct term) isn’t. I also don’t know how a hero image is chosen when a website has multiple images.