 There can be two types of buttons:
24bit BMP's: These ones will be drawn exactly as you did
8 bit BMP's: These ones will allow you to have transparent zones in your buttons (like the ones I placed in the 'leaf' folder). The color you place in the lower left corner will be substituted with your system's solid window color.

(For programmers: the bitmaps are loaded with the LR_LOADRANSPARENT and the LR_LOADMAP3DCOLORS)