Private Function GetPicFiles(ByVal direc As String) As List(Of String)
   'Name/Pfad der Bilddateien aus einem Verzeichnis auflisten 
   Dim PicFileExtension() As String = _
      {"*.bmp", "*.jpg", "*.gif", "*.png", "*.tif"}
   If Not IO.Directory.Exists(direc) Then Return Nothing
   Return _
     My.Computer.FileSystem.GetFiles(direc, _
     FileIO.SearchOption.SearchTopLevelOnly, _
     PicFileExtension).ToList
End Function

  Private Sub form_KeyDown(ByVal sender As Object, _
      ByVal e As System.Windows.Forms.KeyEventArgs) Handles Me.KeyDown
 
      If e.KeyCode = Keys.A Or e.KeyCode = Keys.B Then
         tim.Enabled = False 'Show ggf. anhalten
         Select e.KeyCode
            Case Is = Keys.A
               piclist = GetPicFiles("F:\daten\BilderA")
            Case Is = Keys.B
               piclist = GetPicFiles("F:\daten\BilderB")
         End Select
         'Bilderliste prüfen und ggf. Show wieder starten 
         If piclist IsNot Nothing AndAlso piclist.Count > 0 Then
            tim.Enabled = True
         End If
      End If
  End Sub