tb1a = new Image();
tb1a.src = "p_l_1.gif";
tb1b = new Image();
tb1b.src = "p_l_2.gif";
 
tb2a = new Image();
tb2a.src = "p_n_1.gif";
tb2b = new Image();
tb2b.src = "p_n_2.gif";

function Wechseln(bd,tb)
{
document.images[bd].src = eval(tb + ".src")
}