Many cameras have an orientation sensor inside, so they know which way you are holding the camera and which way should be up. They encode that orientation information into the image file. In the computer, some computers read that orientation correctly and turn the image to make it appear right side up. Some don't (some Macs). Some will make the image appear correctly, but the orientation information hasn't been changed, so it will appear wrong when it gets to the next computer.
Depending on which software application you use for photo editing, there is a tool for turning the image right or left or whatever. Turning it right twice should make your image appear correctly.