var a = 0;
 var tst = ['Sepie.jpg', 'Barvam.jpg'];
 function zmena()
 {
  setTimeout("zmena()", 10000);
  document.logoskoly.src = 'http://zsflc.cz/images/cms/' + tst[a];
  a++;
  if(a == tst.length) a = 0;
 };
 zmena();