А если так ? )))
<div id="div_id" style="display: none;">
<img id="img_id" src="http://javascript.ru/forum/images/ca_serenity/misc/logo.gif" width="990" height="388" />
</div>
<script type="text/javascript">
function getStyle(b, a) {
if (document.defaultView && document.defaultView.getComputedStyle) {
return document.defaultView.getComputedStyle(b, "")[a]
} else if (b.currentStyle) {
return b.currentStyle[a]
}
};
alert(getStyle( document.getElementById('img_id'),'width'))
</script>