Thanks to ashmanix for this installation instruction.

Installation:

Double click to install and it will come up with a warning:

"Apple could not verify libimage-reaction-1.3.0-macos-universal.pkg is free of malware that may harm your Mac or compromise your privacy."

This is because the plugin isn't a signed app. Click the "Done" button and then go to 

"Apple -> System Settings -> Privacy & Security"

Scroll down to Security and click "Open Anyway" where it says 
"libimage-reaction-1.3.0-macos-universal.pkg was blocked to protect your Mac."

A popup will appear:
"Apple is not able to verify that it is free from malware that could harm your Mac or compromise your privacy. Don’t open this unless you are certain it is from a trustworthy source."

Click "Open Anyway" then follow the installation wizard.
