Vista - Merge one bitmap onto another in vb

I load a bitmap from a file and display it in a PictureBox:
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
Dim Fn As String = "C:\MyPath\FirstFile.bmp"
Dim B As Bitmap = Bitmap.FromFile(Fn)
PictureBox1.Image = B
End Sub
This works fine!
But now I want to load a second bitmap and write onto the first bitmap:
Dim Fn1 As String = "C:\MyPath\SecondFile.bmp"
In the second.bmp, the image is surrounded by white pixels, which I want to
make transparent when I merge the images.

How do I do this?

Thanks.