Turn images in 90° steps or mirror them, applying the same angle to a whole batch at once.
A photo taken on a phone often shows up sideways on a computer. The camera did not actually turn the picture — it wrote a note into the file saying it should be displayed rotated, and some software reads that note while other software ignores it. Rotating here in 90° steps turns the pixels themselves, so the image looks the same everywhere you open it.
Flipping is a different operation from rotating. It is what you want for a selfie that came out mirrored, or when you need a subject to face the other way to fit a layout. The one thing to watch is text: flip an image containing words and the words reverse too.
Rotating and flipping do not touch image quality. The colour reduction used to shrink PNGs elsewhere on this site is deliberately not applied here — turning an image should never be the reason its colours changed.
Everything runs inside your browser. Because the image never reaches a server, we cannot store it or look at it. That difference matters when the file is company material or a personal photo.