Why photos end up sideways
Phones record which way they were held in a hidden orientation tag instead of turning the picture itself. Many websites, older programs, printers and messaging apps ignore that tag, so a photo that looks upright on your phone appears sideways elsewhere. Scanned pages fed the wrong way have the same problem.
What happens when you rotate
Choose 90°, 180° or 270° clockwise. The picture is turned for real — the pixels themselves are rotated — so it looks the same in most apps. For 90° and 270°, width and height swap. Nothing is cropped, and the result is saved as a new PNG file without the hidden camera and GPS details.
Choosing the angle
If the top of the picture points left, rotate 90°. If it points right, rotate 270°. If it is upside down, rotate 180°. Several pictures that are wrong in the same way can be rotated together.
Common problems and how to fix them
The PNG is bigger than the original JPG: PNG is lossless; use Compress Image or Image to JPG afterwards if size matters. You need a small tilt correction, such as 3°: this tool turns in quarter turns only; straighten scanned documents with Deskew Scanned PDF. The picture is mirrored rather than rotated: use Flip Image Horizontally.
Good habits
Check orientation before uploading to forms, keep the original photo, and rotate first before cropping or resizing.