kod = new Array(5); 

kod[0] = '<td background="img/bg1.jpg" class="proc">';
kod[1] = '<td background="img/bg2.jpg" class="proc">';
kod[2] = '<td background="img/bg3.jpg" class="proc">';
kod[3] = '<td background="img/bg4.jpg" class="proc">';
kod[4] = '<td background="img/bg5.jpg" class="proc">';

losowa = Math.floor(Math.random() * kod.length); 
document.write(kod[losowa]);