var linkIdPrefix = "link";
var imageIdPrefix = "img";
var imagePostfixOn = "_on";
function swapImageOn(linkId)
{
    var imgElem = document.getElementById(imageIdPrefix + linkId.substr(linkIdPrefix.length));
    var srcParts = splitFilepath(imgElem.src);
    imgElem.src = srcParts[0] + imagePostfixOn + srcParts[1];
}

function swapImageOff(linkId)
{
    var imgElem = document.getElementById(imageIdPrefix + linkId.substr(linkIdPrefix.length));
    var srcParts = splitFilepath(imgElem.src);
    imgElem.src = srcParts[0].substring(0, srcParts[0].length - imagePostfixOn.length) + srcParts[1];
}

function splitFilepath(filePath)
{
    var dotPos = filePath.lastIndexOf(".");
    return new Array(filePath.substring(0, dotPos), filePath.substr(dotPos));
}

function preloadSwapImages()
{
    for (var i in document.images)
    {
        var imgElem = document.images[i];
        if (imgElem.id && imgElem.id.substr(0, imageIdPrefix.length) == imageIdPrefix)
        {
            var srcParts = splitFilepath(imgElem.src);
            var tmpImg = new Image();
            tmpImg.src = srcParts[0] + imagePostfixOn + srcParts[1];
        }
    }
}