function changeImg(mypic,iw,ih){ 
var xw=iw; 
var xl=ih; 

var width = mypic.width; 
var height = mypic.height; 
var bili = width/height; 



var A=xw/width; 
var B=xl/height; 

if(!(A>1&&B>1)) 
{ 
if(A<B) 
{ 
mypic.width=xw; 
mypic.height=xw/bili; 
} 
if(A>B) 
{ 
mypic.width=xl*bili; 
mypic.height=xl; 
} 
} 
}