It'll make the code far easier to read (and less typing) if we create a couple of new functions.

function show_card(id) {
  var image=document.getElementById(id);
  image.src='cards/'+g_cards[id]+'.png';
}
function hide_card(id) {
  var image=document.getElementById(id);
  image.src='cards/back.png';
}
function button_click(colr) {
  Hide('buttons');
  g_n++;
  if (g_cards[1]==g_cards[4]) {
    if (colr=='green') g_score++;
  } else {
    if (colr=='red') g_score++;
  }
  show_card(1);
  show_card(4);
  // wait two seconds
  for (var i=0;i<5;i++) hide_card(i);
  pick0();
  show_card(0);
  // wait three seconds
  hide_card(0);
  move();
  for (var j=4; j>0; j--) g_cards[j]=g_cards[j-1];
  // when move finished
  Show('buttons');
}