Terragen uses proprietary procedural based shaders, there is no way that you can export them for use in another application; with bitmaps you lose the key advantage of procedural based surfaces in that with procedural based surfacing you can move the camera an arbitrary distance form the terrain, and not lose detail, with bitmaps you don't have this.
With the bit-mapped approach (Assuming a game engine is the final use for said image) you need to know how far form the surface of an object it is planned to let the user (Re: Player) get and know that you'd require X resolution for that distance and create your bit map at the required resolution, in you asset creation package in this case TG2.
A small caveat to the above: the problem with bit maps for games (Providing this is the end use) is that they only work a certain distance form the camera before you see pixels; games that are examples Starlancer and Freelancer (Digital Anvil released through Microsoft Game Studios) The Quake Series and Doom 1-3 (Not sure about Doom 4).
Sorry if you know this already, but there you go.
Regards to you,
Cyber-Angel
