Two slutions: one, overlay another image. The overlay image would be transparent except for the region you want to highlight, and with opacity set low enough to still see what is behind it. And two, use the real image as the background image to the aforementioned 'overlay' image (the overlay image must have the overlay region already be translucent instead of using css).
e.g. (version one)
<span class='image_container'>
<img id='base_image' src='base.png' >
<img id='overlay_image' src='overlayimage.png' usemap='#yourmaphere'>
<map name='yourmaphere'>
...
</map>
</span>
.image_container {
position:relative;
}
#overlay_image {
position:absolute;
top:0;
left:0;
opacity:0.5;
filter: alpha(opacity = 50);
/* text-decoration: blink; */ /*optional*/
}
e.g. (version two)
<img id='base_image' src='overlayimage.png' usemap='#yourmaphere'>
<map name='yourmaphere'>
...
</map>
#base_image {
background: transparent url(base.png) no-repeat scroll top left;
}