Setting asp:Image URL using Javascript function

function ChangeImage(imgid) {

            var imgmain = document.getElementById('<%=ImageBig.ClientID%>');
            imgmain.src = imgid.src + "&Height=170&Width=170";
           
            var imagename = "";
            imagename = imgid.src;
            imagename = imagename.substring(imagename.lastIndexOf('/') + 1, imagename.length);
           
            //alert(imgid.src);
        }