在ie8下使用excanvas.js 画图像，内存不回收，怎么办？


<!DOCTYPE html>
<html lang="zh"  >
    <head>
        <meta http-equiv="pragma" content ="no-cache"/>
        <meta http-equiv="Cache-Control" content ="no-cache"/>
        <meta http-equiv="expires" content ="-1"/>
        <meta charset="utf-8" />
        <meta name="viewport" content="width=device-width" />
        
        <title></title>
         <script src="jquery-1.8.0.min.js" type="text/javascript"></script>
        <script src="html5.js" type="text/javascript"></script>
        <script src="excanvas.js" type="text/javascript"></script>
     </head>
    <body>
 
        <canvas id="paperview" style=" border-width:1px;  border-color:Red; border-style:solid;position:relative;padding:0px;margin:0px;"  >
        </canvas>
     
    <script type="text/javascript" language="javascript">
        var paper = null;
        var img = new Image();
        $(function () {            var papercontainer = document.getElementById("paperview");
            paper = papercontainer.getContext('2d');            //设置画板大小
            $("canvas").eq(0).attr("width", 2480);
            $("canvas").eq(0).attr("height", 1752);
                
            //图像下载完成后画图像
            img.onload = function () {
                //清除画板
        paper.clearRect();
        paper.drawImage(img, 0, 0, img.width, img.height);                window.setTimeout(setimg, 2000);
            };
            img.onerror = function () { alert('下载图像失败！'); }
            setimg();
        });        //设置图像
        function setimg() {
        var url="tt.jpg?q=" + Math.random();
            img.src = url;
        document.title =url;
        }
    </script>
 
    </body>
</html>