Are you sure its tiff group 6?

You can use this dotimage function to get the compression used for an image.

We use tiff but only group 4.

[Visual Basic]
Dim comp As Integer = Convert.ToInt32(TiffDecoder.GetTiffTag(259, FileName, 0).Data)
Select Case comp
     Case 1          ' None
     Case 2          ' CCITT modified Huffman RLE
     Case 3          ' CCITT Group 3 fax encoding
     Case 4          ' CCITT Group 4 fax encoding
     Case 5          ' Lempel-Ziv & Welch (LZW)
     Case 6          ' 6.0 Old Style JPEG
     Case 7          ' JPEG DCT compression
     Case 32773  ' Macintosh RLE
     Case 32946  ' Deflate
End Select

Reagrds,
Graham.