Public Shared Function ImageSetBackColor(ByVal ImageStream As Stream, ByVal _
  BackColor As Color, ByVal Format As Imaging.ImageFormat) As Stream
      Dim Image As New Bitmap(ImageStream)
 
      'http://www.vbarchiv.net/tipps/details.php?id=2300
      ' Größe des Bildes
      Dim Width As Integer = Image.Width
      Dim Height As Integer = Image.Height
 
      ' ein neues Bitmap-Objekt in der benötigten Größe erstellen
      Dim newBitmap As New Bitmap(Width, Height)
 
      Using oGraphics As Graphics = Graphics.FromImage(newBitmap)
         ' Hintergrund füllen
         oGraphics.FillRectangle(New SolidBrush(BackColor), 0, 0, Width, Height)
 
         ' transparentes Bild auf den Hintergrund ausgeben
         oGraphics.DrawImage(Image, 0, 0)
      End Using
 
      ImageStream = New MemoryStream
      Image.Save(ImageStream, Format)
      ImageStream.Position = 0
 
      Return ImageStream
   End Function