IIRC Bryce can use 16 bit greyscale images? was some odd way to do that many years ago, but version 6 and on can use them natively?
16 bit greyscale allows for MUCH more variation in height maps as it isn't 8 bit, 8 bit limiting you to 256 shades of grey.

Photoshop lets you use 16 bit images, Paint Shop Pro new versions kind of let you (weird as many functions won't work, grr!)

this picture,
http://www.renderosity.com/mod/gallery/index.php?image_id=1408490&user_id=7541&np&np
the asteroid is built from a terrian using 16 bit image, as 8 bit images due to the size of the object, no matter what you'd do would create obvious "steps" as 256 shades of grey simply isn't enough variance to cover the slow genele slope over most of it, and the edges.
So I built it in Photoshop, using lot of careful gaussian blur and gradients to keep it nice and smooth, exported as 16 bit .tif
:)

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