The link to bright is dead, and my main issue has always been keeping the RGB value at 176 x3 when converting it to the bitmap
I don't know how your software does it, but in gimp you would go to image>indexed to 256 colors. Then you can convert it to bmp, then you will have to go to windows>dockable dialogs>colormap. Then you can use the color picker to highlight the background color in the colormap, then hit edit this color on the bottom left. Set it and that's the color fixed for you.
Btw, there's another thing called interpolation that keeps the picture looking real nice even after sizing it down a ton, this is my "special resizing technique". The hitch is that it will change the color values, including the background, so you have to resize the background as a separate layer without interpolation, and the rest with it.
My advice is to do the art by itself with no background, that is paste the art into a transparent layer, then create a separate one under that and dump it with the gray color, then resize.