In a default OBJ2FACT .mtl file it looks like this:

# map_Kd diffusemap
# map_bump bumpmap
# map_Ks specularmap
# map_Ka ambientmap
# map_D transparencymap

The "#" before each line is a switch. The names are placeholders. Lines that lead with a "#" will be ignored by OBJ2FACT. If you want to assign a texture, you have to remove the switch and you have to replace the default name with the name of the texture file you want to use:

map_Kd My_cool_rgb.image < edited line
# map_Ks specularmap
# map_bump bumpmap

Now OBJ2FACT will ask for the image - if it is not in the same directory as the OBJ - find it, and the textures will be loaded into the right channels.

If you want to assign more than one texture per channel just add a new line with the new texture name:

map_Kd First_rgb.image < edited line - first diffuse texture
map_Kd Second_rgb.image < added line - second diffuse texture
# map_Ks specularmap
# map_bump bumpmap