BREW: ImageMagick Command PNG->BMP

(Install ImageMagick)

for %%i in (*.png) do call convert -background "#ff00ff" %%i -flatten %%i.bmp

Using BMP palettes instead of 24bit color:

for %%i in (*.png) do call convert -background "#ff00ff" %%i -flatten -type "Palette" +matte -colors 256 BMP2:%%i.bmp