How to mirror image files via command line?

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

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

For this particular task convert is probably the best way to go, but for this kind of thing I often use the netpbm library, which is installable (as you would expect) with apt install netpbm. Then

  pngtopnm input.png | pnmflip -lr \
    | (other transformations if desired) \  
    | pnmtopng > output.png

For this task it's overkill, but I often find myself writing one-off scripts to transform or analyze PNM files in peculiar ways that wouldn't be available in convert. This is relatively easy, because PNM is pretty much the simplest imaginable bitmap graphic format.