...
  <script type="text/javascript" language="JavaScript">
  var cursorX, cursorY;
  
  function get_cursor_position (event) {
    cursorX = event.clientX + document.body.scrollLeft;
    cursorY = event.clientY + document.body.scrollTop;  
  }
  
  function item_description (event, header, text) {
    get_cursor_position (event);
    document.getElementById ("message").innerHTML = header + '<br><small>' + text + '</small>';
    document.getElementById ("message").style.left = (cursorX + 1) + 'px';    
    document.getElementById ("message").style.top = (cursorY + 1) + 'px';    
    document.getElementById ("message").style.display = 'block';    
    this.onmouseout = function (event) {
      document.getElementById ("message").style.display = 'none';    
    }
  }
  </script>
...
<div id="message"></div>
<img src="items/potion_health.png" class="item" onmousemove="java script:item_description (event, 'Eliksir zdrowia', 'Przywraca 50% punktów życia');">
...

item_description (event, 'Eliksir zdrowia', 'Przywraca 50% punktów życia');

item_description ('Eliksir zdrowia', 'Przywraca 50% punktów życia');