does a javascript function only run a variable once per refresh?? (switch: case) -

i have script i'm writing runs function change picture when button hit. following script changes picture first time, next time same script run different variables doesn't seem take newest variable account.

<div id="main_img"> <center> <button style="width:100;height:100" onclick="lastpic();"><---</button> <img id="img" src="12.jpg" height=70% width=70%> <button style="width:100;height:100" onclick="firstpic();">---></button>  </div>  <script> var james = document.getelementbyid("img").getattribute('src');  document.write(james);  function firstpic() {     switch (james) {     case "12.jpg":         document.getelementbyid("img").src = "13.jpg";         break;      case "13.jpg":         document.getelementbyid("img").src = "14.jpg";         break;       default:         alert('default case');         break;     } } </script> 

var james = document.getelementbyid("img").getattribute('src'); document.write(james);  function firstpic() {     // new value every time function called     james = document.getelementbyid("img").getattribute('src');      switch (james)     {         case "12.jpg":             document.getelementbyid("img").src = "13.jpg";             break;          case "13.jpg":             document.getelementbyid("img").src = "14.jpg";             break;          default:              alert('default case');             break;     } }