function btnover(a) { var str =
 a.src; a.src = str.replace(/.jpg/,"-o.jpg"); } 
function btnout(a) { var str =
 a.src; a.src = str.replace(/-o.jpg/,".jpg"); }

image1 = new Image();
image1.src = "rondo-prawo-jazdy-mmz-o.jpg";
image2 = new Image();
image2.src = "o-rondo-o.jpg";
image3 = new Image();
image3.src = "kursy-o.jpg";
image4 = new Image();
image4.src = "wymagane-dokumenty-o.jpg";
image5 = new Image();
image5.src = "karta-motorowerowa-o.jpg";
image6 = new Image();
image6.src = "kontakt-o.jpg";