Dim bmp As New Bitmap("C:\meinBitmap.png") 'Zu ladendes Bitmap
Dim Percentage As Integer = 50'Transparenz in Prozent
'Bitmap durchlaufen
For x As Integer = 0 To bmp.Width - 1
   For y As Integer = 0 To bmp.Height - 1
      Dim oCol As Color = bmp.GetPixel(x,y) 'Die Alte PixelFarbe
      bmp.SetPixel(x,y,Color.FromArgb(255 * Percentage / _
        100,oCol.R,oCol.G,oCol.B))'Neue Farbe festlegen
   Next
Next