I have Windows XP and the bmps I cannot open with Zillions are also those I
cannot open with Microsoft Paint. They usually date back to 2000 or 2001,
such as the hex6x6.bmp file for Glinski, or the boards for alf-grandchess,
alf-courier, alf-chinese... as I wrote in a comment for the Omega Chess
zrf.
Anyway, I cannot open with Microsoft Paint the bmps which belong to the
following subdirectories:
Abstract-Shogi/Flip (the bmps in Abstract-Shogi are all right)
Abstract-Shogi-Big, except the King
Abstract-Shogi-Big/Flip, except the King.
Hope this helps.