An image that has a rotation specified in its Exif metadata. JPEG files can have a flag that tells the viewer to flip or rotate the image.
In some older browsers or programs, the image may appear to be sideways or upside-down if the program doesn't support EXIF rotation. For some very old images, the image may to be appear sideways or upside-down because the artist themselves was not aware the image contained an EXIF rotation.
See issue #3692.