ASP.NET : ImageButton Image Doesn't Update inside UpdatePanal

I have an ImageButton inside an UpdatePanal, OnClick event, Some process is preformed on same image. than Image save on same location.

Problem is, The Code Behind Executes Successfully, But result doesn't show on Webpage

ASP Code:

<asp:ScriptManager ID="ScriptManager1" runat="server" />
    <asp:UpdatePanel ID="UpdatePanel1" ChildrenAsTriggers="True" UpdateMode="Always" runat="server">
        <ContentTemplate>  
            <asp:ImageButton  ID="ImageButton1" runat="server" AutoPostBack="true" ImageUrl="~/Sel_Img/test.jpg" OnClick="ImageButton1_Click" />
        </ContentTemplate>
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="ImageButton1" />
        </Triggers>
    </asp:UpdatePanel>

Code Behind

protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
{
   using (Bitmap bitmap = new Bitmap((Server.MapPath(@"/Sel_Img/test.jpg"))))
   Bitmap bmp = Some_Process(bitmap);
   bitmap.Dispose();
   bmp.Save(Server.MapPath(@"/Sel_Img/test.jpg"));
   ImageButton1.ImageUrl = "../Sel_Img/test.jpg";
}

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When i reload Page Manually, Results shown as expected, But Onclick Event the results does not appear , I.E Changes in Image Doesn't Appear.