Lemmy removes EXIF data from pictures, since it includes potentially private information such as phone model, where IRL the pic is from, date, time, etc. But as a side effect, it also removes a tidbit of info that says "rotate this pic when showing it".
The way that I usually solve this* is by editing the picture in Kolourpaint. Either cropping it or resizing it.
*by coincidence to post cat pictures. Yours is making such a silly face :3 love it!
That happens to me on Reddit as well. I only use it on my Mac so it’s usually an HEIC file from my phone I’m posting. If I convert it to a JPEG before it posts in the correct orientation. Lots of things don’t accept heic files so I have a pretty seamless workflow to do it often.