function nextpic() {
var nummer = Math.floor(Math.random()*4);

var bilderpool = new Array("url('gfx/header/sicherheit.jpg')", "url('gfx/header/schienenverkehr.jpg')", "url('gfx/header/luftfahrt.jpg')", "url('gfx/header/kontakt.jpg')");

document.getElementById('header').style.backgroundImage = bilderpool[nummer];
}