[Gimp-user] colour to black and white

[saulgoode at flashingtwelve.brickfilms.com]

Oops, I just realized that what you describe may be attributed to the  
fact that GIMP will display a dark gray/light gray checkerboard to  
indicate transparent regions. Perhaps you have created your lithograph  
correctly but misinterpreted this representation of transparency.

You might try creating a new white layer and placing it at the bottom  
of the layerstack to show the regions where your image is transparent.