The orientation of the picture is stored in the EXIF data of the image.
Some sites, like Facebook, like to remove the EXIF data from the images you upload, which removes the orientation data.
If the site you use does this, you will need to bring the photo into an editor where you can rotate the image left then right then save the image, assuming the editor you choose does a physical rotation of the image. This way the physical orientation will match the EXIF data orientation making the EXIF data not needed for the orientation of the image.