A brief answer to question #1 is: Sometimes we cheat and use 2D graphics programs to make our renders look better. Or you could refer to question two as well, since some models are easier to make outside of Bryce. To answer question #2, which asks "What's compatible?" Well... As far as Bryce obp. files go or .br4/.br5... When it comes to Bryce proprietary files, only Bryce is compatable with Bryce. Also note that Bryce 5 is backward compatable but Bryce 4 is not forward compatable. It's an irritating fact of life sometimes, but don't let that get your hopes down. Why? Because... (as stated previously) Bryce can still export terrains, stones, imported objects, and symetrical lattice objects in formats other programs can use. Bryce can import models created in a decent variety of formats. .3DS, .obj, .dxf and some more are supported by Bryce. Therefore, any program that can save or export in those formats is functionally compatable with Bryce to some degree. These include freeware modeling programs such as Wings3D, Anim8or, Metasequoia, etc. If you find yourself limited by Bryce's way of modeling, this import capability opens up many options. Also note that heightmaps and images planes are your regular .gif/.jpg/.bmp/ etc type files. So any 2D program can come in handy for creating those. So that said, welcome to the club. Experiment. Make images and animations. And most importantly - have fun at it!

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.