var b_over=new Image(); b_over.src="images/b_over.gif";
var b_out=new Image(); b_out.src="images/b_out.gif";
var b_down=new Image(); b_down.src="images/b_down.gif";

function change_pic(mode){
 if(mode=="over")document.images["button"].src=b_over.src;
 if(mode=="out")document.images["button"].src=b_out.src;
 if(mode=="down")document.images["button"].src=b_down.src;
}