Try this

HTML

 <form id="form1" runat="server">
    <asp:FileUpload ID="FileUpload1" runat="server" onchange="Upload()" />
    <asp:Button ID="btnUpload" Text="Upload" runat="server" style="display:none" OnClick="UploadFile" />
    <hr />
    <asp:Image ID="Image1" runat="server" Height="100" Width="100" />
    <script>
        function Upload() {
            document.getElementById("btnUpload").click();
        }
    </script>
</form>

Code

protected void UploadFile(object sender, EventArgs e)
{
    string folderPath = Server.MapPath("~/Files/");

    //Check whether Directory (Folder) exists.
    if (!Directory.Exists(folderPath))
    {
        //If Directory (Folder) does not exists. Create it.
        Directory.CreateDirectory(folderPath);
    }

    //Save the File to the Directory (Folder).
    FileUpload1.SaveAs(folderPath + Path.GetFileName(FileUpload1.FileName));

    //Display the Picture in Image control.
    Image1.ImageUrl = "~/Files/" + Path.GetFileName(FileUpload1.FileName);
}

Screenshot