I saw article on local news page about Kraken. It's optimizer for images which can save some space by compressing them, without visible changes in image quality. I tried it on few files and have to say that for me it's great. Image was compressed from ~800kB to only 300kB :)
It's online tool => https://kraken.io/
But there is one catch - free version can optimize images no larger than 1MB.
So do you use tools for image optimization?
For my sites I use this very fine addon http://www.elkarte.net/community/index.php?topic=1503.0 O:-) O:-) OK its not been tested that much, but it did work well on a old platform that I used. !
It uses Yahoo service to compress the image, lossless, to save space and bandwidth. That and Kraken are good for dynamic content such as attachments, that you don't really control. Also with Google you can use the browsers pagespeed addon to get compressed images as well, there is no API for that that I am aware, but when you run pagespeed on a page you can then get the optimized images.
If its just static content, like icons, backgrounds, sprites, etc, then I use Riot or if its a PNG file to start with then I use the command line program pngslim (its slow but will get you the smallest png image).
After finished product (in my case, the theme), I optimize every single image, to provide fastest loading times :)
My WordPress uses Smush.it automatically. At work I prefer optipng.