After installation, you'll notice the drop-down menu appear above the browser viewport to the right, which will allow you to hide or show all images at once. You also may control the visibility of each individual image by right-clicking on it, where you will see the options "Hide Image" and "Un-Hide Image", depending on the image's current state.
NOTE: This Add-on was designed to be as lightweight and non-intrusive as possible. Its purpose is to control the visibility of images on a SINGLE web page, at the discretion and intention of the user. Please consider this before leaving a negative review because of 'missing features' that are available in many other Add-ons. Simple is good sometimes :)