Why do vertical images flip horizontal after upload? I run into this issue once in a while and I am not sure what it is.  For example I have an image that I get from a non Drupal site or directly from a camera. If I view the image via Preview on my Mac or on a browser from the site it displays vertically. As soon as upload to my Drupal site, it flips horizontally.

To go around this I just open the image on my Mac and just re-save.

I always thought that my computer somehow is rotating the images just for display (based on some logic) but really in the background it is still a horizontal image. But I don't understand why the image displays correctly in the browser from the non Drupal site?
Probably the program looks on your mac to the orientation of the recording itself. Most cameras place information in the photo or it is taken in portrait or landscape. The program that reads the picture can then look at it and put the picture correctly on the screen.