You can't. In HTML anyway :-) In Javascript, you can detect the screen dimensions (or window dimensions), and change the file used for a background - obviously you'd need different standard resolutions as separate image files. For a REALLY fancy way of doing it, you could use PHP to dynamically resize a background image having been given precise dimensions as two variables. Either way, for browser compatibility, you might need to place the background image on a layer at pos 0,0, behind everything else.