When you need the person in the photo looking in the other direction, or you need the top of the image to be the bottom. Mirroring the image (sort of). Right click on the image and follow the menus Tools → Transform Tools → Flip, or use the button on the toolbox.

Using another APOD image [APOD07] I demonstrated all of the flips on this image. You might get bored before it is over ....

The tool used as is (the default) will simply flip the image at its vertical axis.