You could try saving the RAW file as another type of image, BMP would be the best as it is an uncompressed format so doesn't alter the image. You should be able to edit these files in 16bit. Then save the file as a RAW and make sure its the right dimensions(as mentioned in the planetbattlefield tutorial) and 16bit IBM byte order. This should work, though haven't tried it, will give it a go when home.

GIMP with RAW files I think is a no no, then only Plugins for RAWs that I have left to try are Linux and, well, I haven't got that.

Over the weekend I got used to the ingame editor and was pleased, however I don't think its that easy to create a city based map thats not all on one level. I think the city maps are a bit mathematical.

On the Strike at Karkand maps you have those steps running up to the next level of street. You'll find the model files for these but how do you get your map to use them.

Take the following grayscale values:

White 100% Black 0% (Top)

White 75% Black 25% (Mid Top)

White 50% Black 50% (Mid)

White 25% Black 75% (Mid Low)

White 0% Black 100% (Low)

Now all these values are relative to the hightlevel of your map.
e.g.
Heightlevel 100, then Top will equal 100 and Low 0
Heightlevel 700, then Top will equal 700 and Low 0
the other values will the the relative percentage
Heightlevel 700, Mid Low is 25% of 700

Anyway, if you create part of your map with just these grayscale values in a step formation

e.g.
-----
......]
......-----
............]
............-----
..................]

You can import those side walk steps we mentioned eariler and tweak the Heightlevel until the terrain and the model fit.

e.g.
height of the model is 50(just an example) say, so you'd need the heightlevel to be 200

----- Top 200
......]
......----- Mid Top 150
............]{=} <----Step model going up to the next level
............----- Mid 100
..................]
..................----- Mid Low 50
........................]{=} <----Step model going up to the next level
........................----- Low 0

And you'll find that once you've found the heightlevel, you can fit the steps and wall at anylevel Mid, Midlow, mid high.

However on the ingame editor you can only work with a circle shape brush, which unless you want to spend ages with a small circle you really need a flat edge. With a graphics program as I expect the reason why you want to use photoshop this is all quiet simple as long as you keep percentages in mind.