How to transform a white background in an image to a transparent background with The Gimp 1.2 (version 2):

• image -> mode -> rgb
• image -> mode -> decompose -> rgb
• select original image, and press ctrl+c and ctrl+v to make a copy in a new image (let's call the new image the transparancy map image (tmi))
• be sure that the tmi has just 1 layer (merge if necessary)
• tmi: image -> mode -> greyscale
• tmi: image -> colors -> invert
• tmi: new layer (white), mode "overlay"
• tmi: copy that new white layer 3 times (image should get more contrast now) (yes, this is hackish)
• tmi: merge visible layers
• all places that are black in the tmi will be fully transparent in the final image, all places that are white in the tmi will not be transparent at all; if there are places that should not be transparent but they are black (or grey) in the tmi, modify them now (also for places that should be transparent but are white or grey in the tmi)
• tmi: image -> mode -> compose -> rgba. Select red for red, blue for blue and green for green channels, and the tmi for the alpha channel