I think this is a feature request. We have users putting images in posts, and sometimes those images are quite tall, i.e. height much greater than the width. Discourse provides an image preview in the post, but uses the image height within the post height to determine the aspect ratio. An example image looks like this:
Ideally we’d like to present this image so that the image width takes the full post width, but perhaps shows a ‘More…’ link at the bottom. The ‘image viewer overlay’ (not sure what type, mfp css classes) struggles with this format too, in that it squishes the file info at the bottom.
So yep, probably a bit unusual but thought I’d throw that out there anyway. If anyone has any simple custom CSS then we could give that a go too.