How to mirror image files via command line?
Solution 1:
From quick reading of this, apparently convert calls this option -flop for horizontal mirroring, and -flip for vertical. All I needed to do was
convert -flop input.png output.png
Solution 2:
If you want to overwrite in-place and you have a ton of image files in the same folder, mogrify from the ImageMagick suite seems to be the easiest way to achieve this:
# mirror in the vertical axis:
mogrify -flip *.jpg
# mirror in the horizontal axis:
mogrify -flop *.jpg