Posunutí obrázku po stisknutí klávesy

var fish = "small";
var big_position_x = 0;
var big_position_y = 0;
var small_position_x = 0;
var small_position_y = -50;
var move_up = false;
var move = false;
// var big = "images/big.gif";
// var small = "images/small.gif";

if(move) {
  // Předělej obrázek stojící ryby v DIVu na pohyb
  if(fish == "small" && move_up) {
    small_position_y = small_position_y + 5;
    el = document.getElementsById(fish).style;
    el.marginTop = small_position_y;  
  }
  
  if(fish == "big" && move_up) {
    big_position_y = big_position_y + 5;
    el = document.getElementsById(fish).style;
    el.marginTop = big_position_y;  
  }
}


if(!window.addEventListener) {
  window.addEventListener = function (type, listener, useCapture) {
    document.body.attachEvent('on' + type, listener);
  }
}

function move_up(e) {
  if(!e) e = window.event;
  if(e.keyCode == 87) {
    move = true;
    move_up = true;
    e.returnValue = false;
    return false;
  } else {
    return true;
  }
}

function stop_up(e) {
  if(!e) e = window.event;
  if(e.keyCode == 87) {
    move = false;
    move_up = false;
    e.returnValue = false;
    return false;
  } else {
    return true;
  }
}

window.addEventListener("keydown", move_up, false);
window.addEventListener("keyup", stop_up, false);

<script type="text/javascript" src="ryby.js"></script>

<div id="small">
<img src="images/man.png">
</div>