First of all, great plugin.
I noticed that in the "wp-content" folder there's a folder called "ewww" that contains several of the optimization applications. These same files exist within the "wp-content/plugins/ewww-image-optimizer" folder.
Should this folder and the contents within actually exist directly within "wp-content" considering the files are redundant? It seems like the installer forgot to delete them.
Also, I uninstalled the plugin (just out of curiosity) to see if the "ewww" folder and its contents would be deleted, but they were not removed. I had to delete it manually.
Everything I've reported occurs on Linux and Windows.