Hi Willem,
This rang a bell for me, and I managed to track down two older threads in the forum about a similar issue.
I suspect the problem is not the size of your image. Instead, it is likely that the original image was edited, and it has some EXIF metadata that describes an image rotation. Unfortunately, AtoM does not currently know how to parse this type of EXIF metadata, and so it is ignored during the derivative creation process, leading to the derivatives looking like they are oriented wrong.
Here are the old threads I found:
And here is the bug ticket we filed based on it:
I will bring this issue to the attention of our Maintainers, so they are aware that this is an ongoing issue impacting users, and so they can consider it for the next release.
In the meantime, if you look at the second forum thread above, there are some suggestions for a workaround in the meantime to address the issue. Let us know how it goes!
Cheers,