アルファ付きの画像とデスクトップを綺麗に重ねるには？

    <System.Runtime.InteropServices.DllImportAttribute("gdi32.dll")> _
    Private Shared Function DeleteObject(ByVal hObject As IntPtr) As Boolean
    End Function

    <Runtime.InteropServices.DllImport("gdi32.dll", ExactSpelling:=True, SetLastError:=True)> _
    Public Shared Function SelectObject(ByVal hDC As IntPtr, ByVal hObject As IntPtr) As IntPtr
    End Function

    <System.Runtime.InteropServices.DllImport("user32.dll", ExactSpelling:=True, SetLastError:=True)> _
    Public Shared Function UpdateLayeredWindow( _
        ByVal hwnd As IntPtr, _
        ByVal hdcDst As IntPtr, _
        <System.Runtime.InteropServices.In()> ByRef pptDst As System.Drawing.Point, _
        <System.Runtime.InteropServices.In()> ByRef psize As System.Drawing.Size, _
        ByVal hdcSrc As IntPtr, _
        <System.Runtime.InteropServices.In()> ByRef pptSrc As System.Drawing.Point, _
        ByRef crKey As Int32, _
        <System.Runtime.InteropServices.In()> ByRef pblend As BLENDFUNCTION, _
        ByVal dwFlags As Int32 _
       ) As Boolean
    End Function

    <System.Runtime.InteropServices.StructLayout(Runtime.InteropServices.LayoutKind.Sequential, Pack:=1)> _
     Public Structure BLENDFUNCTION
        Public BlendOp As Byte
        Public BlendFlags As Byte
        Public SourceConstantAlpha As Byte
        Public AlphaFormat As Byte
    End Structure

    Private Const WS_EX_LAYERED As Integer = &H80000
    Private Const WS_BORDER As Integer = &H800000
    Private Const WS_THICKFRAME As Integer = &H40000
    Private Const AC_SRC_OVER As Byte = 0
    Private Const AC_SRC_ALPHA As Byte = 1

    Private Const ULW_COLORKEY As Int32 = 1
    Private Const ULW_ALPHA As Int32 = 2
    Private Const ULW_OPAQUE As Int32 = 4

    Protected Overrides ReadOnly Property CreateParams() As System.Windows.Forms.CreateParams
        Get
            'Set WS_EX_LAYERED
            Dim cp As System.Windows.Forms.CreateParams
            cp = MyBase.CreateParams
            cp.ExStyle = cp.ExStyle Or WS_EX_LAYERED
            cp.Style = cp.Style And (Not WS_BORDER)
            cp.Style = cp.Style And (Not WS_THICKFRAME)
            Return cp
        End Get
    End Property

    Public Sub SetBitmap(ByVal SrcBmp As System.Drawing.Bitmap)
        Dim g_sc As System.Drawing.Graphics
        Dim g_mem As System.Drawing.Graphics
        Dim hdc_sc As IntPtr
        Dim hdc_mem As IntPtr
        Dim hbmp As IntPtr
        Dim hbmp_old As IntPtr
        Dim blf As New BLENDFUNCTION

        g_sc = System.Drawing.Graphics.FromHwnd(IntPtr.Zero)
        g_mem = System.Drawing.Graphics.FromImage(SrcBmp)
        hdc_sc = g_sc.GetHdc()
        hdc_mem = g_mem.GetHdc()
        hbmp = SrcBmp.GetHbitmap(Drawing.Color.Black)
        hbmp_old = SelectObject(hdc_mem, hbmp)

        blf.BlendOp = AC_SRC_OVER
        blf.BlendFlags = 0
        blf.SourceConstantAlpha = 255
        blf.AlphaFormat = AC_SRC_ALPHA

        Dim r As Boolean
        r = UpdateLayeredWindow( _
            Me.Handle, _
            hdc_sc, Me.Location, New System.Drawing.Size(SrcBmp.Width, SrcBmp.Height), _
            hdc_mem, New System.Drawing.Point(0, 0), _
             0, blf, ULW_ALPHA)
        SelectObject(hdc_mem, hbmp_old)
        DeleteObject(hbmp)
        g_mem.ReleaseHdc(hdc_mem)
        g_sc.ReleaseHdc(hdc_sc)
        g_mem.Dispose()
        g_sc.Dispose()

        If Not r Then Throw New ApplicationException(System.Runtime.InteropServices.Marshal.GetLastWin32Error().ToString)
    End Sub