Did some more testing. The cropping happens in the uploading of the image.
for example when I attach an image that is
1428×699
wordpress creates folowing images:
image_1280x626.jpg
(Large – porportianally same and follows my max ‘large’ preset of 1280×720)
image_1280x320x156.jpg
(thumbnail -proportionally same and follows my thumbnail rule -max 320×180)
image_480x234.jpg
(medium, again follows logical rules)
and then the mysterious
image_900x100.jpg
This is the header image size – but why does it get used on the attachment page?
