Public Sub Main()
  Dim myImage As Image = Image.FromFile("C:\image.bmp") 'укажите файл, или изображение
  Dim myEncoder As System.Drawing.Imaging.Encoder = System.Drawing.Imaging.Encoder.Quality
  Dim myEncoderParameter As System.Drawing.Imaging.EncoderParameter
  Dim myEncoderParameters As New System.Drawing.Imaging.EncoderParameters(1)
  Dim myImageCodecInfo As System.Drawing.Imaging.ImageCodecInfo
  myImageCodecInfo = GetEncoderInfo(System.Drawing.Imaging.ImageFormat.Jpeg)
  myEncoderParameter = New System.Drawing.Imaging.EncoderParameter(myEncoder, CType(75L, Integer))'75 - качество изображение, от 0 до 100
  myEncoderParameters.Param(0) = myEncoderParameter
  myImage.Save("C:\kbyte.jpg", myImageCodecInfo, myEncoderParameters)
End Sub

Private Function GetEncoderInfo(ByVal format As System.Drawing.Imaging.ImageFormat) As System.Drawing.Imaging.ImageCodecInfo
  Dim j As Integer
  Dim encoders() As System.Drawing.Imaging.ImageCodecInfo
  encoders = System.Drawing.Imaging.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 'GetEncoderInfo