addEvent(window, "load", Roll);

function Roll()
{
    var overMark = "_on";
    var outMark = "_off";
    var overReg = new RegExp(overMark + "(\.[^\.]+)$");
    var outReg = new RegExp(outMark + "(\.[^\.]+)$");

    var imgs = document.images;
    var preloads = new Array;

    for(var i=0, s=imgs.length; i<s; i++)
    {
        if(!outReg.test(imgs[i].src))
            continue;

        var overSrc = imgs[i].src.replace(outReg, overMark+"$1");
        preloads[i] = new Image;
        preloads[i].src = overSrc;

        addEvent(imgs[i], "mouseover", function() {
                                this.src = this.src.replace(outReg, overMark+"$1");
        });

        addEvent(imgs[i], "mouseout", function(e) {
                                this.src = this.src.replace(overReg, outMark+"$1");
        });
    }
}