How to optimize and compress JPEG images with jpegoptim on Linux

You can optimize JPEG images on the Linux command line (= reduce the size to load images faster).
In order to achieve that, you can make use of the 'jpegoptim' application.
On Ubuntu/Debian systems:

apt-get-install jpegoptim

To optimize the JPEG file:

jpegoptim $file
  or
jpegoptim --size=1024 $file    (target 1Mb size)