jQuery.fn.simpleImageRollover = function(preload) {
    
    if (preload) {
        this.each(function() {
            var i = new Image;
            i.src = this.src;
        });
    }
    
    this.hover(
        function() { this.src = this.src.replace(/\.(\w+)$/, '_h.$1'); },
        function() { this.src = this.src.replace(/_h\.(\w+)$/, '.$1'); }
    );

}

$(function() {

    $('#nav img, .rollover').simpleImageRollover();

});