Certain cameras and cameras on phones embed internal information in the image file about the orientation, landscape or portrait, that the picture was taken. When the picture is shown, the program showing the information may use the orientation info or not. If the picture is sideways, most likely the orientation information is not being used.

How to fix: Not an easy fix. I upload my images to my PC to make corrections and then save it in the correct orientation by rotating the picture if necessary. Then I can safely upload the picture to a website.