var camArr = new Array("analog1", "analog2", "ip");

function refresh_cameras(thumb)
{
  for (var i in camArr)
  {
    var src = 'http://www.taborsigma.cz/camera2.php?device=' + camArr[i] + '&id=' + Math.random(); 
    
    if (thumb)
    {
      src += '&thumb=1';
    }
      
    document.getElementById('cam_' + camArr[i]).src = src;
  }
}