Im not sure what to do about this either. I had my chasm vertical so top down didnt show any relief at all. But it looks too flat. I am unsure if that is how it would look in real life or whether I should have faked it a bit more for artistic sake to give some depth.

In Gimp I regularly use up to 20K pixel sized images. Not sure if there is a limit. I know in the past I used to get problems after about the 18K mark on my old paint software and in any case I think if your using images bigger than about 12K its time to break them up into tiles. I normally only use 20K images in Gimp so that I can make the tiles for the zoom maps.

We did the MeDem map which is 40K pix square in tiles. It was not easy using tiles but we found that we had to. I think there are some apps which work in tiles in the background but the main app will page them in and out for you so you dont notice. In fact I think its possible photoshop does this. I do quite a lot of image work in scripts and automated methods. Most my current city map is automated scripts and then I can run the same script on different tiles and then piece it all back together again - not that I am this time around.