var pic = new Array();
var active = new Array();
var rnd = new Array();
rnd[1] = 1;
rnd[2] = 2;
active[1] = true;
active[2] = true;

pic[1]="img/rnd/r1.jpg";
pic[2]="img/rnd/r2.jpg";
pic[3]="img/rnd/r3.jpg";
pic[4]="img/rnd/r4.jpg";
pic[5]="img/rnd/r5.jpg";
pic[6]="img/rnd/r6.jpg";
pic[7]="img/rnd/r7.jpg";

function replaceImage(img) {
  do {
    num = Math.floor(Math.random()*7)+1;
  } while (active[num] == true);
  active[rnd[img]] = false;
  active[num] = true;
  rnd[img] = num;
  document.getElementById('rnd'+img).src = pic[num];
  setTimeout("replaceImage('" + img + "')", 5000);
}

setTimeout("replaceImage(1)", 2500);
setTimeout("replaceImage(2)", 5000);