I use this plugin for years on a lot of sites, and it always works!
Just delivers a great value for all websites which are frequently updated with new images by non-tech editors (who have no clue of image sizes and compression rates).
Also, check out the cloud/pro version of the plugin. For really a small amount of money, you get reduced server load and the ability to shrink overly big image sizes automatically.
Thank you Shane Bishop for this great contribution!Steffen Dressler