ill explain what it is but you don't need it and ill explain that too

alpha is what tells a picture veiwer what's visable and what's not... depending on what program your using its done differently
you don't need it at all if you do this..
skip using a bmp in favor of a png and you just make the pic on a transparent backround and the backround when loaded into rco edit will remain transparent even if it looks white on your pc...