
That's expected. The first sentence describing the bitmap resource in the
MenuButton section of the "Athena Widget Set - C Language Interface"
documentation states

      bitmap      A bitmap to display instead of the label.

One way round this might be to make use of the button's backgroundPixmap
resource, but remember that you, or a user of your application, won't be able
to set it from a resource file. Also, the button won't automatically be the
size of the bitmap. The bitmap will be tiled if necessary, or you could set the
button's dimensions to match its size.

Jim