<script type="text/javascript" src="iuembed.js"></script>
<script type="text/javascript">
var iu = new ImageUploaderWriter("ImageUploaderID", 610, 500);
//...other params...
iu.addParam("ProgressDialogCloseButtonText", "Close");
//...other params...
iu.writeHtml();
</script>

getImageUploader("ImageUploaderID").setProgressDialogCloseButtonText(value);
value = getImageUploader("ImageUploaderID").getProgressDialogCloseButtonText();