Background Image on Image Map

[aparth]

I've been trying and failing to create an image map that contains a tiled background image underneath the main image.

My image map is 600px in height. I want the main image (img_bottom.jpg) to only cover the bottom 100px of the map (and the background to fill the rest of the space).

The height of the main image will change, which is why I want to leave the empty space as a tiled background image.

#my_map {
    display: block;
    height: 600px;
    width: 1000px;
    margin: 0px auto;
    margin-bottom: 10px;
    position: relative; 
    top: 0px; 
    left: 0px; 
    border: 2px solid #51595C;
    border-top: none;
    background: url(../images/ob09_5.jpg) repeat top left;
}
 
<map name="my_map" id="my_map"></map>
 
<img name="imap" id="imap" src="./images/img_bottom.jpg" ismap usemap="#my_map" />
 
 

Any suggestions on how to achieve this on all browsers?