//图片按比例缩放
var flag=false;
function DrawImage(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;
			}
		}else{
			if(image.height>iheight){ 
				ImgD.height=iheight;
				ImgD.width=(image.width*iheight)/image.height; 
			}else{
				ImgD.width=image.width; 
				ImgD.height=image.height;
			}
		}
	}
}