<script runat="server">
Sub choose_image(Sender As Object, e As EventArgs)
   image1.Src = select1.Value
End Sub
</script>

<!DOCTYPE html>
<html>
<body>

<form runat="server">
<select id="select1" runat="server">
   <option value="smiley.gif">Smiley</option>
   <option value="angry.gif">Angry</option>
   <option value="stickman.gif">Stickman</option>
</select>
<input type="submit" runat="server" value="Display image"
OnServerClick="choose_image">
<br><br>
<img id="image1" src="smiley.gif" runat="server" width="32" height="32" />
</form>

</body>
</html>