I got this to work in the end by running a script in the child HTML document to detect the image load. This then triggers a 'fade function' in the parent document which fades up the containing iFrame.
<script type='text/javascript'>
var img = new Image();
img.onload = function(){
window.parent.fadeFn();
}
<?php echo"img.src='".$fileNameVar."'"?>
</script>