imagemagick

ImageMagick is a well-known CLI tool that provides image processing capabilities. It can handle hundreds of different image formats and gives you a programmatic interface to create, edit, compose, and convert images. I personally use the convert command very often to resize an image (e.g., convert -resize 300x300 -quality 100 source.png out.png) or convert between different formats. ImageMagick also has the ability to produce a visual diff between two images, by using its compare & montage commands.