The board files are actually ZIP archives that contain a gif file among others. Edit the file name to add a .zip suffix and edit the gif file. Not sure what other things you need to do to the other files to make it work?
Most the files inside the .zip that are not .gifs (/.pngs/.jpgs?) are text files. LOS data is binary, I believe. That may be the only pure binary file inside a map .zip that vasl uses.