FWIW, here is an entry from Microsoft's Support Online that addresses the problem:

BUG: BMP Turns Black on a Disable Button under NT 4.0

>I have noticed a strange problem with bitmaps assigned to the picture property for command buttons. When the button is disabled when an application is running, the bitmap is completely grayed out in a block (even the bitmap outline is not visible). This happens only on machines with Windows NT; Windows 95 machines display correctly. Does anyone else have this problem, and if so, is there a workaround? Thanks.