I'm don't know if you have a relevant medical condition (if that's the right term), but you really should be able to divide by two in your head. I can't imagine how this would be faster for you to write, it wastes bandwidth and is dependent on Javascript being enabled. It's just not a good idea.
Anyway..
PHP Code:
<script type="text/javascript">
<!--
function halfImageSize()
{
var images = document.getElementsByTagName("img");
for (i = 0; i < images.length; i++)
{
images[i].width /= 2;
images[i].height /= 2;
}
}
// -->
</script>
<body onLoad="halfImageSize();">