Not long ago we changed a term used in our product. Domains are now Sources.
With this change we expect to give a clearer idea of how rethumb works and what this means in practical terms.
When you change to a paid plan you have access to new features on the requests that you can make, but rethumb needs a way to differentiate your requests from those using a free plan (which apply by default to all sources). We do this by asking you to register in which source the images that you want to process are hosted.
A source is something like this:
So all images hosted on domain.com and sub.domain.com are avaible to process using the features of your plan.
For now we only accept HTTP(S) sources but in the future we plan to allow new protocols like FTP, S3 and anonymous locations. Authentication is also something that we want to work on.
Our FAQ have more information on this subject.