[Nekromancer]

<form action="" method="get" name="user" id="user">
    <select id="user_id">
        <option value="1" /> User 1
        <option value="2" /> User 2
        <option value="3" /> User 3
        <option value="4" /> User 4
        <option value="5" /> User 5
    </select><br />
    <input type="button" id="but" value="Change" />
</form>
 
<img src="img/user_1.jpg" id="imgUser" width="350">
<script>
    var but = document.getElementById("but");
    onload = f_ChangeUse;
     
    if (but.addEventListener) {
        but.addEventListener("click", f_ChangeUse, false);
    } else {
        but.attachEvent("onclick", f_ChangeUse);
    }
     
    function f_ChangeUse() {
        var val = document.forms["user"].user_id.value;
        document.getElementById("imgUser").src = "img/user_" + val  + ".jpg";  
    }
</script>