图片超过DIV宽度自适应

.main_con img
{
    max-width: 610px;
    height: auto;
    cursor: pointer;
    border: 0px double #cccccc;
    padding: 2px;
    zoom: expression( function(elm) {    
    if (elm.width>610)                 
    {  var oldVW = elm.width; elm.width=610;                                
   elm.height = elm.height*(610 /oldVW);                         
    }         
   elm.style.zoom = '1';            
   } (this));
}