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.

The tool used as is (the default) will simply flip the image at its vertical axis. If you double click on the button, up will pop the tool options diaolog window. Using this dialog or keeping pressed the Ctrl key it is possible to choose horizontal flip instead. All possible flips are displayed in this page.