How to optimize already uploaded images? Is it possible to compress or optimize images that are already uploaded to the server? What about a method to optimize the images on upload?

Goal:


Take all existing images, compress them, and then save them with the same file name and directory so that the image URLs are not broken. 
On upload of an image, optimize it so we don't have to repeat #1 above.
The common approach is not to modify the uploaded original image, but to calculate additional image variations and serve those to your visitors. All image styles created in /admin/config/media/image-styles are automatically compressed, using the settings form /admin/config/media/image-toolkit