To truly recreate a grayscale heightmap in a raster program you need to spend a long time tweaking the colors of your terrain with minute adjustments in color. Tiny tiny adjustments that have almost no visual effect. Therefore that makes it hard to do. When I was really obsessive about such things I'd import my grayscale into Terragen, have that render the image, then go back to my grayscale and tweak for a while, then repeat. It got to be too time-consuming and really helped cure my obsessiveness.