Choose a quarter turn or a flip and the picture is redrawn that way. Two things are worth knowing. The rotation is real: the pixels move, so every program that opens the file sees it the right way up, rather than depending on an orientation tag that some viewers honour and others ignore. And because the file is redrawn, its metadata does not come along — the GPS position and camera details are dropped as a side effect.
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A real one. The image is drawn onto a fresh canvas at the new angle and encoded from there, so the pixel grid itself has turned. The alternative — writing an EXIF orientation flag — is much cheaper and much less reliable: web browsers, chat apps and older image viewers disagree about whether to honour it, so the same file appears upright in one place and sideways in another.
Because it is re-encoded. A JPEG that goes through this comes out slightly different in size and, strictly speaking, slightly lower in quality, since JPEG loses a little each time it is written. Choose PNG output if you would rather not lose anything; the file will be larger.
It is dropped. Redrawing the image keeps only the pixels, so location, camera model, serial number and the embedded thumbnail all go. That is usually welcome and occasionally not — if you were relying on the capture date, note it down before you rotate.
Yes, and it has to: a 1000 by 500 picture becomes 500 by 1000. Tools that forget this draw the turned image onto a canvas of the original shape and quietly crop the edges off, producing a file that is perfectly valid and missing part of the picture.
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