A photograph that appears sideways usually is not: cameras record which way up they were held and leave the picture itself alone, and every so often something downstream ignores that note. The fix is to turn the pixels themselves, so there is nothing left to ignore. The same applies to a scan that came out upside down, or a picture that needs mirroring because the text in it reads backwards.
One at a time. It is shown the way your camera meant it to be seen.
Quarter turns in either direction, and a flip along either axis.
The rotation is written into the pixels, so nothing downstream can misread it.
Step by step, with screenshots: How to rotate an image
Because the picture is not sideways — the camera stored a note saying which way up it was held, and the two programs disagree about whether to read it. Modern browsers and phone galleries honour it; older software, some upload forms and a few printing services do not. Turning the pixels themselves removes the disagreement entirely.
The rotation itself moves pixels without altering them, but saving a JPEG afterwards re-compresses it, so there is a small generational cost. It is not visible once, and it is worth avoiding doing repeatedly to the same file.
A rotation turns the picture; a mirror reflects it. They look similar on something symmetrical and are completely different on anything with text or a face in it. Mirroring is what you want for a selfie that reads backwards, or a scan taken through the back of a page.
Not here — the turns are quarter turns. Correcting a slightly tilted horizon means rotating by an arbitrary angle and then cropping the corners that fall outside the frame, which is an editing job rather than a conversion one.
Free Online Image Optimizer