When the image is large and you change the dimensions, it’s automatically cropped because of the lightbox CSS.
Even if you resize it:
Small images can be posted with the wanted aspect ratio.
Maybe there’s a very hacky way to be found, such as using an authorized class for the <img> tag, like avatar:
<img width="376" height="50" src="https://meta.discourse.org/secure-uploads/meta/original/4X/b/e/6/be6f2ea64830fe0aaec6c192f3b39c38ce5f179c.png" class="avatar">
But it forces a border-radius.
The original is square and forces a lightbox:
If you’re an admin, I’d say the easiest way is to use a custom wrap and define a CSS rule. For example:
[wrap=ratio]
!image|500x300](upload://raEVqkaMNqIXQ9YcsiZuYefukFu.png)
[/wrap]
[data-wrap="ratio"] .lightbox-wrapper img {
object-fit: unset;
}
I wouldn’t be surprised if there was a way to force the aspect ratio as a user, though, or even an easier way as an admin. Maybe don’t even use a wrap and force unset on object-fit in a lightbox div, but that might mess up the regular layout. I don’t know.