The plugin uses Javascript on the client to determine the device’s screen size. So, if you use it on a desktop browser you will see no changes, because the screen there is big enough.
But if you open your website from a mobile device, smartphone or tablet, the screen size will be detected. Then a smaller image will be served. However, the image will have the same name as the original in the client!
You can see the smaller images being created in the /wp-contents/ai-cache directory.
Also you can test via:
Test with a tool like http://www.webpagetest.org/, but first make sure you set the “Emulate Mobile Browser” setting in the “Advanced Settings” > “Chrome” tab.
