用GDI+放缩图片后，无法再添加水印

Question

我用如下程序片段处理图片。在未指定新的width和height时，添加水印的程序片段可以成功执行，并输出我想要的加水印后的图片。但是一旦我指定了width或者height后，也就是说resize的程序片段被执行后，添加水印的程序片段依然能够被顺利执行，但是输出的图片能够成功被resize，但是不能加水印。请教一下这是什么原因，怎样解决，谢谢！            If IsNothing(Request("image")) Then Throw New Exception("Image file name must be provided!")
            Dim strSourceImage As String = Request("image").Replace("/", "\").ToLower()
            Dim iWidth As Integer = 0
            Dim iHeight As Integer = 0
            If Not IsNothing(Request("w")) Then iWidth = CInt(Request("w"))
            If Not IsNothing(Request("h")) Then iHeight = CInt(Request("h"))
            Dim strActions As String = Request("action")
            Dim strAction As String()
            If Not IsNothing(strActions) Then strAction = strActions.ToLower().Split(",")            If strSourceImage.Length < 6 Then Throw New Exception("Invalid Image file name or path!")
            Dim strExt As String = Helper.GetExtension(strSourceImage)
            If strExt <> "jpg" And strExt <> "bmp" And strExt <> "gif" And strExt <> "png" And strExt <> "jpeg" Then Throw New Exception("Only image format is acceptable!")            Dim strPath As String = ""            If strSourceImage.Substring(0, 1) = "\" Then strPath = Helper.SiteRootPath() & strSourceImage
            If strSourceImage.Substring(0, 4) = "http" Then
                strPath = Server.MapPath("~/") & Config.TempDir.Substring(1) & System.Guid.NewGuid().ToString() & "." & strExt
                Dim myWebClient As New WebClient()
                Response.Write(strSourceImage.Replace("\", "/") & "<br>")
                myWebClient.DownloadFile(strSourceImage.Replace("\", "/"), strPath)
            End If
            If strPath = "" Then strPath = Server.MapPath(strSourceImage)            Dim bmpImage As New Bitmap(strPath)
            Dim gConvas As Graphics = Graphics.FromImage(bmpImage)            Dim f As ImageFormat
            Dim strContentType As String = "image/jpeg"
            Select Case strExt
                Case "jpg"
                    f = ImageFormat.Jpeg
                    strContentType = "image/jpeg"
                Case "jpeg"
                    f = ImageFormat.Jpeg
                    strContentType = "image/jpeg"
                Case "gif"
                    f = ImageFormat.Gif
                    strContentType = "image/gif"
                Case "png"
                    f = ImageFormat.Png
                    strContentType = "image/png"
                Case "bmp"
                    f = ImageFormat.Bmp
                    strContentType = "image/bmp"
                Case Else
                    f = ImageFormat.Jpeg
                    strContentType = "image/jpeg"
            End Select            'Resize image
            If iWidth > 0 And iHeight > 0 Then
                If bmpImage.Width / bmpImage.Height > iWidth / iHeight Then
                    iHeight = CInt(iWidth * bmpImage.Height / bmpImage.Width)
                Else
                    iWidth = CInt(iHeight * bmpImage.Width / bmpImage.Height)
                End If
                gConvas = Graphics.FromImage(bmpImage)
                bmpImage = ResizeImage(bmpImage, iWidth, iHeight, f)
            End If            '加水印
            If bmpImage.Width >= 200 And bmpImage.Height >= 200 Then
                Dim bmpWaterMark As Bitmap = New Bitmap(Helper.SiteRootPath & ConfigurationManager.AppSettings("WaterMark"))
                bmpWaterMark.MakeTransparent(Color.White)
                gConvas.SmoothingMode = SmoothingMode.HighQuality
                gConvas.DrawImage(bmpWaterMark, 20, 20)
            End If            'Special action
            If Not IsNothing(strAction) Then
                For i As Integer = 0 To strAction.Length - 1
                    Select Case strAction(i)
                        Case "gray"                        Case "roundcorner"                        Case "frame" '必须最后进行加相框的处理，因为当所有基本处理运行完毕后才有必要放大图像并加相框                        Case Else                    End Select
                Next
            End If            'Output image
            Response.ContentType = strContentType
            bmpImage.Save(Response.OutputStream, f)            'Remove temp files
            Helper.CleanTempDir()