BMP file to excel with transaparent background

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VB

Visual-Studio

VS2010

I'm trying to import an image to excel. I did import the image, however I want the image's background to be transparent. When I tried this code:

bm_dest.MakeTransparent(bm_dest.GetPixel(1, 1))

the background image changed from white to grey.

Here's the whole code:

Dim oRng1 As Microsoft.Office.Interop.Excel.Range
       Dim F1 As Image
       Dim bm_source As Bitmap
       Dim scale_factor As Single = 0.175
       oRng1 = xlWorkSheet.Range("A8")
       F1 = SignaturePictureBox.Image
       bm_source = New Bitmap(F1)
       Dim bm_dest As New Bitmap(CInt(bm_source.Width * scale_factor), CInt(bm_source.Height * scale_factor))
       Dim gr_dest As Graphics = Graphics.FromImage(bm_dest)
       gr_dest.DrawImage(bm_source, 0, 0, bm_dest.Width + 1, bm_dest.Height + 1)
       bm_dest.MakeTransparent(bm_dest.GetPixel(1, 1))
       Clipboard.SetDataObject(bm_dest, False)
       xlWorkSheet.Range("A43").Select()
       xlWorkSheet.PasteSpecial(Format:="Bitmap")
       bm_dest.Dispose()
       bm_source.Dispose()
       gr_dest.Dispose()

P.S. I am a beginner with VB