function downimage(imgd,maxwidth,maxheight){  
    var image=new image();  
    image.src=imgd.src;  
    if(image.width>0 && image.height>0){  
        var rate = (maxwidth/image.width < maxheight/image.height)?maxwidth/image.width:maxheight/image.height; 
        if(rate <= 1){           
            imgd.width = image.width*rate; 
            imgd.height =image.height*rate; 
        } 
    }  
}