If you have access to Dreamweaver I would suggest adding your image along with centering up your frame there for the time being.
I haven't worked out how to centre the page as yet but as I have to open it in dreamweaver anyhow to add some simple script required for my site, its only a minor addition to the process.
Quark is excellent in giving you the freedom to create a site in a way designers feel the most creative and using the master page keeps the site all tight.
However an extention for these options would be a welcome addition to the experience.