ImageMagick

Tool for graphic editing.

brew install imagemagick

compare pic-a.jpg pic-b.jpg pic-compare.jpg

# information about an image
identify --verbose pic-a.jpg

# compose is frequently used for watermarking
compose wm.png painting.png painting-wm.png

# mogrify modifies the original file
mogrify -rotate "90>" pic.tiff

# creating thumbnails
for i in *.png; do
  convert $i -resize 400x300 `basename $i .png`.jpg;
done