I think it all depends on what you are selling or who your market is.
If selling a service, I would say that having a picture helps build trust and also creates instant rapoort with your audience.
I don't think it truly matters whether the picture is in the header or just in the sales page itself, just that it's there "somewhere".
I created a blog for a friend of mine and even though he's no "Brad Pitt", the guys got a HUGE fanfare of over 500-600 subscribers to his newsletter.
Here's the link:
http://heartsandhumor.com
As you can see he's no Heartbreaker -- (just a regular guy) and he gets traffic like CRAZY! (in fact so much that I've had to extend his bandwidth on my hosting service five times).
I will also say here that I once owned a traffic website that was just puttering along until I added my picture to it. Once added, the website took off like a rocket.
I guess this would be a great thing to test, but I can tell you that when visiting a strange site, I like to know who I'm dealing with.