previouson = new Image();
previouson.src = "images/previous_on.gif";
previousoff = new Image();
previousoff.src = "images/previous_off.gif";

nexton = new Image();
nexton.src = "images/next_on.gif";
nextoff = new Image();
nextoff.src = "images/next_off.gif";

// links image functions
function imgAct(imgName) {
    document[imgName].src = eval(imgName + "on.src");
}

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