I used the template for Books and uploaded cover photos of books. The images were taken on my phone in portrait mode and were uploaded from the mobile app on that same phone. However, after upload, the are turned 90 degrees.
As a workaround, I can download the images from the browser to my Mac (where they appear in portrait again), rotate them 4 times (360 degrees), and re-upload. Then they show up in portrait as expected.