endorphin the portrait phone is tall and takes up more screen than the landscape photos.
That has always been the case since flarum was launched. I'm not sure whether that's a flarum default or a decision that the devlopers have made.
It possibly works well like that on a mobile because the screen is small, but on desktop I fix that with some user css:
max-height: 400px !important;
So you get this rather than a very tall portrait image:
Ideally, it should be supplemented by an option to display the image at full resolution (and perhaps, to include a 400px version of the image in the initial display, as that particular image is a massive 2,340px × 4,160px which is much more than is necessary for the initial preview) but both of those would have to be done in the server, not by css.