That was really clarifying, thanks. I tried to blur the grayscale with Photoshop in order to deal with terracing, but besides that white plateau would keep plain, that's not really what I wanted to do. I thought on making some scale-adjusted, more realistic relief (the scale of the image I uploaded is around 1 [px] : 12.5 [km], so the whole image dimensions are around 5940x4600 [km]) with some geomorphological characteristics like erosion patterns, but I'd need to dispense of a lot of detail in order not to spend a lot of time on it doing all manually. With most of fast techniques I've seen so far I'd have problems deciding where I want to make wrinkled relief (e.g in the plateau), leaving plain those flats in the center.