Branded html email template in OOTB form submission

Hi,

We are using AEM 6.4, and are using plain text emails upon form submission in the OOTB "Send PDF via email" Submit Action. However, we would like to use a branded HTML email template.

We have a template ready to go, but have no idea as to best place to put it and how to bring in graphics such as our logo. Also, the default template in libs/fd/dashboard/templates/email is set up as a .txt file with subject and message lines included. Do we need to encode like this for OOTB?

Thanks!