Many digital cameras today do not just save an image, but also save various meta data in the Exif standard. This data includes information about the position of the camera when the image was taken (such as vertical or horizontal). However, some image programs use this data to rotate the image when displaying it while others don't, leaving the user to face inconsistent behaviour.

This can be fixed with the tool exiftran; it automatically rotates all images according to the Exif data, which it discards afterwards. It is also very easy to use for mass conversion:

# apt-get install exiftran
$ find -print0 | xargs -0 exifautotran

This tool might be shipped by other distributions under a different name. Fedora, for example, calls it fbida.