There is no current options in RC settings, and it didn't matter how much training we provided to our users, they would always mess up the email and cause unwanted cover pages and extra image pages (from signatures and other images) being faxed out. This was just too unprofessional and caused confusion for our fax recipients. So we ended up having to setup our own fax-email account internally (
firstname.lastname@example.org) and then we run an automated tool (that we built) on one of our office computers / servers that picks up these emails and relays them to the RingCentral email address without any subject or email body. As a bonus, we also have our automated tool save copies of all the outgoing faxes so that we have a searchable archive that we can reference when / if a recipient ever inquires about a fax they received, we can lookup who sent it and have an exact copy of what was sent.