function initRollOverImages(){
    var image_cache = new Object();
    $("img.swap").each(function(i){
        var imgsrc = this.src;
        var dot = this.src.lastIndexOf('.');
        var imgsrc_on = this.src.substr(0, dot) + '_over' + this.src.substr(dot, 4);
        image_cache[this.src] = new Image();
        image_cache[this.src].src = imgsrc_on;
        $(this).hover(
            function() { this.src = imgsrc_on; },
            function() { this.src = imgsrc; });
    });
}

$(document).ready(initRollOverImages);