There is an ASCII-Art mode in The Gimp, but that does more than you want (it can handle greyscale and color).

Image::Magick has a perl interface so you can get the image information (width and height) then loop through the pixels.

www.wotsit.org has links to the different file formats. There are a couple of BMP formats depending on the windows version, or it may be an X bitmap, you were not very clear there. Once you know the format you can get the dimensions and then play games with pack and unpack.

If this is an infrequent thing and you don't care about extreme speed then I would go the Image::Magick route.

-ben