//图片按比例缩放
var flag=false;
function ReDrawImage(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;
         }
         }
     }
}