I just started using images in one of my Memento databases and I found what seems like the same problem. What I found is that the thumbnail on the list view is sideways but the image in the record itself is correct - is that what you're seeing? After some fooling around, I found that I only had the problem with images taken with my phone's camera (Samsung Galaxy Note 5) but images from other sources looked fine. The workaround I'm using, which is a pain in the butt, is to take a screenshot of the camera image and use that instead of the original picture. It works, but like I said, it's a pain in the butt. Thankfully I didn't have too many pictures taken with the camera's phone.
Hope this helps.