Optimise JPEG images.

# To optimise a set of JPEG images, retaining all associated data:
jpegoptim image1.jpeg image2.jpeg imageN.jpeg

# To optimise JPEG images, stripping all non-essential data:
jpegoptim --strip-all image1.jpeg image2.jpeg imageN.jpeg

# To force the output images to be progressive:
jpegoptim --all-progressive image1.jpeg image2.jpeg imageN.jpeg