jQuery(document).ready(function () {
    var $ = jQuery;
    $("img.hover_image").hover(
        function () {
            match = /^(.+)\.(jpe?g|png|gif)$/i.exec($(this).attr('src'));
            if (match) {
                $(this).attr('src', match[1] + ".on." + match[2]);
            }
        },
        function () {
            match = /^(.+)\.on\.(jpe?g|png|gif)$/i.exec($(this).attr('src'));
            if (match) {
                $(this).attr('src', match[1] + "." + match[2]);
            }
        }
    );
});