Quoted from oga83:
This is a good hint !
As the images are 4-bit encoded, you can use a 16 level palette.
Although I did not test it, the one below should give good results (low nibble is just ignored to convert each color to 4-bit)
I have downloaded gimp and cannot figure out how to load or setup the palette as suggested. Can anyone help out with a tutorial for dummies (me). thx.