When flipping helps
Mirroring fixes selfies and front-camera photos that came out reversed, and it is a quick way to make a subject face into a layout instead of out of it. Remember that any text in the picture becomes unreadable once mirrored.
How the result is written
The image is redrawn on a canvas with the transformation applied and saved as a new file, so the change is baked into the pixels rather than stored as a tag. Rotating by 90 or 270 degrees swaps the width and height; flipping keeps the dimensions the same. Nothing is re-sampled, so no sharpness is lost beyond the re-encoding of the output format itself.
Batch handling
The same transformation is applied to every file you add, which makes it quick to correct a whole set of photos from one shoot. Results can be downloaded together as a ZIP archive.