DropDown list doesn't post back when linked to Update Panel.

Hi,
I have an Image and a DropDown menu to select the size of the image. Here are the controls and the Page_Load sub:
When I change the DropDown selection, the page posts back with the new Image height:
<asp:DropDownList ID="DropDownImg" AutoPostBack="true" runat="server" /> <asp:Image ID="FrontImage" ImageUrl="~/img/nature.jpg" runat="server"/> Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load If Not Request.IsAuthenticated Then FormsAuthentication.RedirectToLoginPage() End If Try Dim userName As String = Request.Cookies("cookieUserName").Value If Not IsPostBack Then loggedUser.Text = "User: " & userName DropDownImg.Items.Add(New ListItem("100% (4524 x 3393)", "3 ...