var img=new Array();
img[0]=new Image();img[0].src="com_images/btn_home01.jpg";
img[1]=new Image();img[1].src="com_images/btn_home02.jpg";
img[2]=new Image();img[2].src="com_images/btn_contact02.jpg";
img[3]=new Image();img[3].src="com_images/btn_contact01.jpg";

function change_image(name,num) {
document.images[name].src=img[num].src;
}