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!