When you place a glyph (bitmap-picture) on a button, the color of the pixel
in the picture's lower-left corner will be used as the transparent color. All
the pixels of that color will be transparent, the button's surface will be visible
at these spots.
If you don't want any transparent parts on your button
bitmap, make sure to use a color in the lower-left corner that is not used anywhere
else in the bitmap.