var MyTest = document.getElementById("upload_img").files[0]; var reader = new FileReader();reader.readAsDataURL(MyTest);reader.onload = function(theFile) {　　var image = new Image();   image.src = theFile.target.result;   image.onload = function() {   　　alert("图片的宽度为"+this.width+",长度为"+this.height);   };};

其中，id为“upload_img”的标签如下：