Functionality
Before starting, let me make one thing clear – unlike most other image optimization plugins, it optimizes images on your server itself. This can be extremely convenient but if your server isn’t powerful enough and you’re bulk optimizing images, it can probably crash your server.
Functionality wise, it is chock full of options. After installing head over to settings > Ewww image optimizer and explore the default options.

This dashboard conveniently shows the system status as well as optimization options. Most default options should work for all users. Just be careful with setting lossy as the compression type as your jpgs/png will have lower quality.
There is also an option to include/exclude folders for optimization. This is particularly good if you’re keeping originals on the server for a photography website.
In the resize tab, there is an option to automatically resize images when uploaded to the media library. There are also other options to disable optimization on particular thumbnail sizes.
Keep in mind that some options need the premium version of this plugin to be able to be activated.
Lastly, as a bonus there is an option to convert image to WebP. This is a new fomat developed by Google which further compresses image sizes. Just be aware that not many browsers support WebP at the time of this review.
Optimizing images
Head over to the media library and select the list view(click on the list icon just below ‘Media Library’. Now click on ‘Optimize Now!’ button in the Image Optimizer column and there you go, you have optimized that image.
If you want to optimize all your images at the same time, select ‘Bulk Optimize’ under Sidebar > Media

Running this may take a while so just sit back and get a coffee or something until it finishes the job. If you run it again, it will skip images already optimized which can significantly cut down optimization time.