谷歌火狐 上传图片预显示

不懂得什么原理，能用就好

<div id="CourseImg">
</div>
<input type="file" onchange="previewImage(this);"/>
<script type="text/javascript">
    function previewImage(file){
        var MAXWIDTH = 99;
        var MAXHEIGHT = 133;
        var div = document.getElementById("CourseImg");
        
        if (file.files && file.files[0]) {
            div.innerHTML = '<img id=imghead>';
            var img = document.getElementById('imghead');
            img.onload = function(){
                var rect = clacImgZoomParam(MAXWIDTH, MAXHEIGHT, img.offsetWidth, img.offsetHeight);
                img.width = rect.width;
                img.height = rect.height;
                img.style.marginLeft = rect.left + 'px';
                img.style.marginTop = rect.top + 'px';
            }
            var reader = new FileReader();
            reader.onload = function(evt){
                img.src = evt.target.result;
            }
            reader.readAsDataURL(file.files[0]);
        }
        else {
            var sFilter = 'filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=scale,src="';
            file.select();
            var src = document.selection.createRange().text;
            div.innerHTML = '<img id=imghead>';
            var img = document.getElementById('imghead');
            img.filters.item('DXImageTransform.Microsoft.AlphaImageLoader').src = src;
            var rect = clacImgZoomParam(MAXWIDTH, MAXHEIGHT, img.offsetWidth, img.offsetHeight);
            status = ('rect:' + rect.top + ',' + rect.left + ',' + rect.width + ',' + rect.height);
            div.innerHTML = "<div id=divhead style='width:" + rect.width + "px;height:" + rect.height + "px;margin-top:" + rect.top + "px;margin-left:" + rect.left + "px;" + sFilter + src + "\"'></div>";
        }
    }
    
    function clacImgZoomParam(maxWidth, maxHeight, width, height){
        var param = {
            top: 0,
            left: 0,
            width: width,
            height: height
        };
        if (width > maxWidth || height > maxHeight) {
            rateWidth = width / maxWidth;
            rateHeight = height / maxHeight;
            
            if (rateWidth > rateHeight) {
                param.width = maxWidth;
                param.height = Math.round(height / rateWidth);
            }
            else {
                param.width = Math.round(width / rateHeight);
                param.height = maxHeight;
            }
        }
        
        param.left = Math.round((maxWidth - param.width) / 2);
        param.top = Math.round((maxHeight - param.height) / 2);
        return param;
    }
</script>