This works:

        Public Function SaveAsHighQualityJpegFile() As Boolean
            Dim loEncoderParameters As System.Drawing.Imaging.EncoderParameters = New System.Drawing.Imaging.EncoderParameters(1)
            Dim loImageCodecInfo As System.Drawing.Imaging.ImageCodecInfo = Nothing
            Dim loCodec As System.Drawing.Imaging.ImageCodecInfo

            loEncoderParameters.Param(0) = New System.Drawing.Imaging.EncoderParameter(System.Drawing.Imaging.Encoder.Quality, 100)

            For Each loCodec In System.Drawing.Imaging.ImageCodecInfo.GetImageEncoders()

                If loCodec.MimeType = "image/jpeg" Then
                    loImageCodecInfo = loCodec
                    Exit For
                End If

            Next

            oImage.Save(cFilePath, loImageCodecInfo, loEncoderParameters)
            Return True
        End Function

Where oImage is my System.Drawing.Bitmap object.

When doing that, it saves in JPG. If I call the regular System.Drawing.Bitmap Save() method, it saves in PNG. So, the disk space ratio is at least 4 times smaller.