Why are my images sideways?
Certain image files contain EXIF metadata that control the image orientation - this is more common with images that were captured on mobile devices and then reoriented. This orientation may be supported on some devices but ignored by others - resulting in your images sometimes appearing sideways.
The easiest workaround is usually to rotate the photo in image editing software, save it, then rotate it back to how you want it and then save it again. There are other more technical ways to remove the EXIF metadata, such as the one outlined here.