Replace exisisting file with another file

Dim files As HttpFileCollection = HttpContext.Current.Request.Files
        Dim uploadfile As HttpPostedFile = files("RemoteFile")
        Dim filePath As String
        filePath = System.Web.HttpContext.Current.Request.MapPath("Images1\\" + DirName) & "\" & uploadfile.FileName
        Dim filename As String = (uploadfile.FileName).ToString()
        Dim fileExtension As String = Path.GetExtension(uploadfile.FileName)
        Dim DocumentName As String = uploadfile.FileName
        If (ContentType = "text/html") Then
            If (fileExtension = ".tiff") Then
        uploadfile.SaveAs(filePath)
  'Construct a bitmap object using original image so you can draw this object on the target bitmap
                Dim OrgImg As New Bitmap(filePath)
                'Construct a bitmap object for the stamp image
                '-----------------------------------------------
                'Create Bitmap object of specific width and height
                Dim ImgStamp As New Bitmap(400, 100)
                'Obtain Graphics object to perform graphics opration
                Dim g As Graphics = Graphics.FromImage(ImgStamp)
                'Use drawString method of the graphics object to write text on target bitmap
                Dim currtime As DateTime = DateTime.Now.ToString()
                g.DrawString(currtime, New Font("Arial", 15, FontStyle.Bold), _
                    New SolidBrush(Color.Red), 15, 15)
 
                'Save this bitmap using its save method as .Tiff,.jpg or any other image
                ImgStamp.Save(DirPath & "\MyStamp.Tiff")
                'You can optionally specify the ImageFormat using ImageFormat Class
                'ImgStamp.Save(YourPath & "\MyStamp.Tiff",_
                'System.Drawing.Imaging.ImageFormat.Tiff)
                '-----------------------------------------------
                'Dim StampImg As New Bitmap(StampImagePath & "\MyStamp.Tiff")
                 Dim StampImg As New Bitmap(DirPath & "\MyStamp.Tiff")
        'Create target bitmap to draw both original image and stamp image.
        Dim TarImg As New Bitmap(OrgImg.Width, OrgImg.Height + StampImg.Height)
        'Obtain a Graphics object from & for that Bitmap
        Dim gr As Graphics = Graphics.FromImage(TarImg)
        'Draw the original image first on the target bitmap using graphics object
        gr.DrawImage(OrgImg, 0, 0, OrgImg.Width, OrgImg.Height)
        'Now draw the stamp image on the target bitmap using graphics object
gr.DrawImage(StampImg, 0, OrgImg.Height, StampImg.Width, StampImg.Height)
        'Save the target bitmap as image file
              TarImg.Save(filename) ' This is where I am getting a Generic Error in GDI+
                files = Nothing
                filePath = Nothing
            Else
            End If
        End If
    End Sub
 
End Class

                                  

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