window._hover_onload = window.onload;

window.onload = function () {
    var hovers = document.getElementsByTagName('img');
    for (var i = 0; i < hovers.length; i++)
    {
        if (hovers[i].className == 'button')
        {
            hovers[i].onmouseover = function () {
                this.src = this.src.replace('/default/', '/hover/');
            };
            hovers[i].onmouseout = function () {
                this.src = this.src.replace('/hover/', '/default/');
            };
        }
    }
    if (window._hover_onload)
        window._hover_onload();
};