// 封装
function getNaturalSize (DomElement) {

    var natureSize = {};

    if(window.naturalWidth && window.naturalHeight) {

        natureSize.width = DomElement.naturalWidth;

        natureSizeheight = DomElement.naturalHeight;

    } else {

       var img = new Image();

        img.src = DomElement.src;

        natureSize.width = img.width;

        natureSizeheight = img.height;

    }

    return natureSize;

}

// 使用

var natural = getNaturalSize (document.getElementById('img')),

    natureWidth = natural.width,

    natureHeight = natural.height;