It is explained in this section of the user guide, but short version is to make an edit to the photo first, such as cropping it.
/nosanitize
https://help.counter.social/en/latest/posting-content-advanced.html#attaching-media-files
Or just take the photo in landscape mode, like God intended. 👍
@th3j35t3r I see phase's toot as an example :(