jquery中如何获取图片真实大小

$("<img/>") // Make in memory copy of image to avoid css issues  在内存中创建一个img标记
    .attr("src", $(&#39;&#39;#imgid").attr("src"))
    .load(function() {
        pic_real_width = this.width;   // Note: $(this).width() will not
        pic_real_height = this.height; // work for in memory images.
    });

var theImage = new Image(); 
theImage.src = $(''#imgid").attr( "src"); 
alert( "Width: " + theImage.width); 
alert( "Height: " + theImage.height);