This is somewhat subjective, but IMHO, it would depend on which is more important to purpose of the page - the picture or the text. You could always do the image as a semi-transparent background and put the text over top. Just a thought.
What shows above the fold is the most important part... that's where you get a user or see one leave. With LARGE pics be sure they are optimized for both size and speed... even with broadband slow to load might lose a viewer.
No matter how fast computers become, they are never fast enough... particularly when we get used to the most recent speed increase!
That said, if the PIC is the context/idea, then by all means make it most prominent. Since navigation is mentioned then there's more to offer, and I would make it easier to get to the rest of the stuff than create a landing page that might lose some eyeballs.