[NTG-context] Building a page based on exact location of graphical elements (PDF)

What is the simplest way to build a page based on positioning of external images? Some of these images are page-sized (so take up an entire page, though I might want to overlay some other text boxes and MP graphics). I’ve been trying to find a good explanation/introduction into layers (which I think ConTeXtgarden says I should use) but haven’t found one. G