It happens to me all the time when I post a photo of a paper drawing. I don't know how to stop it.

It seems like you’re taking these as actual photos with a camera.
What do you use for these? If it’s a phone, it could very well be that it thinks you’re taking the photos in the horizontal orientation instead of the vertical one. Fixing that may be as simple as going into a basic image editor and rotating the photo, something which I think most image viewers that come packaged with desktop and phone operating systems can do with one button nowadays.
If it’s being uploaded to FurAffinity as a horizontal picture and is somehow being rotated by the site, I don’t know how to solve that, but you could try clicking “update submission file” and dropping the same picture with the correct rotation again.
That should also work for the first case when it comes to fixing your rotated work, as well.

That might be the case, I do use my phone to take pictures of my drawings. I might try what you said, thanks.