Your account starts in a free plan, at any moment you can subscribe to a paid plan for one year. These plans allow you to use special features for one year, after that you can manually renew your subscription or it will be move to a free plan.
After subscribing to a paid plan you can register one or more sources, something like yoursite.com.
Then, you can start making requests to rethumb using images hosted on yoursite.com.
The paid plan features are only available for the images hosted on the sources that you register.
Check paid plan features here: http://rethumb.com/pricing.