Inbound internet faxes are received at the internet fax providers' servers on regular phone lines, then forwarded to you by email. This means that there are two points where security compromises are introduced by the service:
1. On the internet fax provider's fax server - There is no solution to overcome compromise number one. You either trust the provider not to abuse your faxes or you don't.
2. In the email transmission from the internet fax provider to you - Since your fax traverses the public internet, it is no more and no less secure than any other email transmission. If privacy is a very important issue to you, then select a provider who will encrypt faxes that are forwarded to you.