js代码如下：
var flag=false;
function DrawImage(ImgD,h,w){
    var image=new Image();
    var iwidth = w; //定义允许图片宽度
    var iheight = h; //定义允许图片高度
    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;
    }
    }
}

调用上面的JS
<img onload="DrawImage(this,250,250)">