I just fired up chrome and tested right click and it works fine (actually does not display context menu).
1) It does not work if you are logged into your WordPress page. Admins can right click the images.
2) It does not work for images within iframes or the image is presented as other than a background or an IMG tag.
3) It does not work if there is an html tag covering the image such as a transparant image that is used to fade or obscure the image.
4) If you website is using CloudFlare or other caching, it may take time for the web page to hit your browser. Make sure that you refresh the site and clear any caching before you test.
Please log off of your WordPress site before you try to test the plugin. Please clear all caches and reload the page.