Use CSS position: absolute;
followed by top: 0px; left 0px;
in the style
attribute of each DIV. Replace the pixel values with whatever you want.
You can use z-index: x;
to set the vertical "order" (which one is "on top"). Replace x
with a number, higher numbers are on top of lower numbers.
Here is how your new code would look:
<div>
<div id="backdrop" style="z-index: 1; position: absolute; top: 0px; left: 0px;"><img alt="" src='/backdrop.png' /></div>
<div id="curtain" style="z-index: 2; position: absolute; top: 0px; left: 0px; background-image:url(/curtain.png);background-position:100px 200px; height:250px; width:500px;"> </div>
</div>