var image = new Image();
var that = this;
image.onload = function(){
if ("naturalWidth" in image) { // 现代浏览器
that.width = image.naturalWidth;
that.height = image.naturalHeight;
} else { // IE6/7/8
that.width = image.width;
that.height = image.height;
}
image = undefined;
delete image;
}