The only way that you could reliably do that would be to only put the body in your HTML and load the rest of the content dynamically (i.e., using AJAX / DOM) after the body loads.
In practice, this is ridiculously difficult, and it defeats the purpose of what you're trying to accomplish, since the people that visit your page will be much more interested in your content than your background.
If you had a dark background, and your text were white, then the text wouldn't be visible until the background image loaded (though the side effect of that is that, in the User's mind, the page would take a lot longer to load, since from his perspective, *NOTHING* loaded until your background did).