After subscribing to a paid plan you have to add a source. Only images hosted on those sources are processed using the plan features.
I subscribed to a Pro plan that allows me to process images with 6 MB in size.
My images are online at http://my-site.com, so I register "my-site.com" in the dashboard to be able to process the images hosted at that domain.
In this way I can process a large image (E.G.: 5 MB, http://my-site.com/large_img.jpg) that the free plan won't allow.
By entering the domain "my-site.com" as a source to use paid features all subdomains are automatically accepted as well: