However, I noticed two things:

The image mask seems to be supported only for `:mask 'heuristic'. This
determines the image mask by looking at the four corners of the
image. This is o.k. for almost any icon in the tool bar. But to get
its job done correctly in the general case, xpm.el needs to specify a
certain colour as the image mask, like: `:mask '(heuristic (0 0 0))'. 
This is so, because an XPM image may specify transparancy in arbitrary
places. For example: it could be a square containing a transparent
circle in the middle.

Unfortunately Emacs on MS Windows does not deal with 
`:mask '(heuristic (0 0 0)'. I generated the result in the attached
image by making a small change in xpm.el in order test your fix,
otherwise the icons would appear on a black background. But as I said
above, this is wrong in the general case. So could you implement this?
Or at least make Emacs on MS Windows ignore the part after
`heuristic'. This should do for most cases as far as icons are
concerned.

Another thing is, that it seems that the :ascent attribute does not
work; gamegrid.el needs to specify `:ascent 'center' for images.

    Oliver
-- 
Oliver Scholz               27 Brumaire an 211 de la Révolution
Taunusstr. 25               Liberté, Egalité, Fraternité!
60329 Frankfurt a. M.       http://www.jungdemokratenhessen.de
Tel. (069) 97 40 99 42      http://www.jdjl.org