Convert PNG to 32bpp Bitmap

I need to create a bitmap object that either preserves the alpha channel of a png or adds one if the png has no alpha channel. This:

Dim a = New Bitmap( _
    My.Application.Info.DirectoryPath & "\a.png")
            Dim b As New Bitmap(a)

Will alter the colors or alpha values. I use the Image.Save method described here to convert pngs to bmps:
http://msdn.microsoft.com/en-us/library/ytz20d80.aspx
Now I just need to convert pngs to 32bppArgb bitmaps instead.

The msdn example saves to a file, but I save to a stream instead. There is only one ImageFormat.Bmp so what do I do?

This is the 'GetEncoderInfo' function from msdn:

Private Shared Function GetEncoderInfo(ByVal format As ImageFormat) As ImageCodecInfo
        Dim j As Integer 
        Dim encoders() As ImageCodecInfo
        encoders = ImageCodecInfo.GetImageEncoders()

        j = 0
        While j < encoders.Length
            If encoders(j).FormatID = format.Guid Then 
                Return encoders(j)
            End If
            j += 1
        End While 
        Return Nothing 

    End Function

Can I write that to convert to 32bppArgb bitmap instead?