Ecco subito il codice:
------------------------------------------------------------
Imports System.IO.Pach
Imports System.Drawing.Imaging

Public Function ConvertBMP(ByVal BMPFullPach As String,_  ByVal imgFormat As ImageFormat) As Boolean

Dim bAns As Boolean
Dim sNewFile As String

Try
  Dim objBmp As New Bitmap(BMPFullPach)
  sNewFile = GetDirectoryName(BMPFullPach)
  sNewFile &= GetFileNameWithoutExtension(BMPFullPach)
  sNewFile &= "." & imgFormat.ToString
  objBmp.Save(sNewFile, imgFormat)
  bAns = True
cath
  bAns = False
End Try
Return bAns

End Function  
------------------------------------------------------------
Spero che questo codice possa servirvi ...