I can suggest the CentOS image.
The basic droplet will do for WHMCS, and you can increase it if you feel it slow on low on resources, it all depends on the load you put to it, just pay attention to the software requirements. If you would like any more information on WHMCS or need help in determining if your server meets the requirements, please contact them.
WHMCS is the Client Management, Billing & Support Solution for Web Hosts. It is used by most hosting providers and is a great tool for managing customers, billing, etc.