Convert picture to 32bit

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VB

I need to convert a picture to 32bpp, or ARGB, in visual basic. That is, I need to add an alpha channel to a picture that doesn't have one. This is my attempt:

Dim a = New Bitmap( _
 My.Application.Info.DirectoryPath & "\ping.png")
         Dim b As New Bitmap(a.Width, a.Height)
         Dim g As Graphics = Graphics.FromImage(b)
         g.DrawImage(a, 0, 0)
         PictureBox1.Image = b
         g.Dispose()

This resizes the image somehow - makes it bigger but then crops it to the original's size.