to answer my own questions...
No, there is no photoshop plugin for xpm files ...at least not one that is easily found.
After banging my head against google an adobe's site for an hour,
I tried converting my bmp's to xpm's form maya, using this:
http://www.highend3d.com/maya/downloads/me.../misc/2490.html
but it seems to not work either.
I tried to use imgcvt to do it , with teh result " imgcvt: unknown output image type - xpm."
in the help files, xpm is not listed as one of the supported formats
In my googling, it seems that this problem has happened to other as well:
They make a custom icon as a bmp, and use it successfully
Then they move workstations or install new maya versions, and the custom icons disappear
trying to manually load them to a shelf =" not a valid icon"
Seems odd that what once worked now does not. any idea why Maya 'forgets' that the bmps are actually valid?
seems odd that people have suggested (and scripted) converting to xpm with imgcvt, when it doesn't actually work.
My new solution : screw it! I'll just use text labels.