Michel,
Just to clarify things here. The HTML support isn't a feature of PlanetPress, it's your email client(such as Outlook) that will read the email's content as HTML, and interprete it accordingly.
You can put anything you want in the body including variable data in the various HTML/CSS fields, PPress won't treat the message any differently if you put HTML or not. Whether the email will actually be read as HTML and display the attached image, and whether it fully supports CSS, all depends on which email client you use, and how you set it up, and it's feature/limitations. (for example, you have a better chance of getting everything fully working if you use Outlook's Inbox rather than Windows Live's Inbox)
Regards,
Rapha