Floyd's absolutely right. One of my laptops absolutely refuses to load images larger than screen size -- the remaining image gets filled with random memory garbage when you try to display it; you have no guarantee that loading any images in larger than that will work. IIRC it's to do with the way drivers interact with DirectX's surfaces but the specifics elude me.
To do what you want to do reliably, you'll need to chop the image up into smaller bits and tile them traditionally.
To do what you want to do reliably, you'll need to chop the image up into smaller bits and tile them traditionally.