function get_pic() {
  clear();
  Shuffle(g_ind);
  g_answer_id=Random0(4);
  var pic_n=g_ind[g_answer_id];
  if (g_used.includes(pic_n)) {
    g_k++;
    if (g_k<500) {
      get_pic();
    } else {
      alert("Sorry - can't find another picture");
    }
  } else {
    g_used.push(pic_n);
    var el=document.getElementById('pic');
    el.src='images/'+pic_n+'.jpg';
    for (var i=0;i<5;i++) {
      var el=document.getElementById(i);
      var pic_n=g_ind[i];
      el.textContent=g_pics[pic_n];
    }
  }
}

Why 500? No good reason - it just needs to be big!

Make sure you try this out before proceeding. You should get that alert message after you've seen all your pictures.