reSmush.it is a free image optimization API. It compresses the size of your images, making them — and therefore your website — significantly faster to load. reSmush.it image style optimizer is a Drupal module that provides an image style effect with this image optimization capability.

Project page: https://www.drupal.org/project/resmushit
More info and demo: http://www.vacilando.org/article/resmushit