//图片按比例缩放
var flag=false;
function imageSize(ImgD,iwidth,iheight) {
    //参数(图片,允许的宽度,允许的高度)
    var image=new Image();
    image.src=ImgD.src;
    if (image.width>0 && image.height>0) {
        flag=true;
        if (image.width/image.height>= iwidth/iheight) {
            if (image.width>iwidth) {  
                ImgD.width=iwidth;
                ImgD.height=(image.height*iwidth)/image.width;
            } else {
                ImgD.width=image.width;  
                ImgD.height=image.height;
            }
            ImgD.alt=image.width+"×"+image.height;
        } else {
            if (image.height>iheight) {  
                ImgD.height=iheight;
                ImgD.width=(image.width*iheight)/image.height;        
            } else {
                ImgD.width=image.width;  
                ImgD.height=image.height;
            }
            ImgD.alt=image.width+"×"+image.height;
        }
    }
} 
//调用：<img src="图片" onload="javascript:DrawImage(this,100,100)">