function changeImage1(){
    document.btn_bn1.src="img/btn_bn2.gif";
}
function changeImage2(){
    document.btn_bn1.src="img/btn_bn1.gif";
}

function changeImage3(){
    document.btn_bn2.src="img/btn_bn2.gif";
}
function changeImage4(){
    document.btn_bn2.src="img/btn_bn1.gif";
}