if (document.images) {
    img1on=new Image();
    img1on.src="images/diet-m1y.gif";
    img1off=new Image();
    img1off.src="images/diet-m1.gif";

    img2on=new Image();
    img2on.src="images/diet-m2y.gif";
    img2off=new Image();
    img2off.src="images/diet-m2.gif";

    img3on=new Image();
    img3on.src="images/diet-m3y.gif";
    img3off=new Image();
    img3off.src="images/diet-m3.gif";

    img4on=new Image();
    img4on.src="images/diet-m4y.gif";
    img4off=new Image();
    img4off.src="images/diet-m4.gif";
}

function imgOn(imgName) {
    if (document.images) {
        document[imgName].src=eval(imgName + "on.src");
    }
}

function imgOff(imgName) {
    if (document.images) {
        document[imgName].src=eval(imgName + "off.src");
    }
}