Using a 2D pre-built image as a background in Bryce
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Hopefully you've read the Moodflow workflow tutorial. If not, go ahead and read that first. If so, lets move on.  I am going to assume you have some decent experience with Bryce.

As stated, I try to work from back to front, so lets start with a background in Bryce:
In Bryce, you can do it one of two ways: create the background then and there using Bryce's presets, OR use a previously generated IMAGE as the background (which is my preferred method).  Most of my scenes are done this way, so we'll concentrate on that.

Start with a new scene, and NO BRYCE CLOUDS! Disable the sunlight too. However, keep the haze option available. 

Figure 1 - Base "Blank" Bryce Screen

I have previously created a 2D image in Terragen to use as the background.  This image sets the lighting and mood of the rest of the image. I call this a billboard image (its also called an alpha plane by other artists), but I'll be using the term 'billboard' image through my tutorials.
 

In Bryce, I create a billboard image by creating a 2D rectangle with the background image APPLIED AS A TEXTURE!  I usually turn the textures ambience to 100%, as this image will in some ways BE the lighting of the image (though it really won't project light onto foreground elements). Additionally, I don't want to subdue the colors and lighting of that image (since there is no other light in the scene yet). So 100% will bring it back to its brilliance. Otherwise, a low ambience setting will make the image dull and washed out. There are exceptions to this, but these will be noted later.  Sunlight should be disabled, or otherwise it will over-brighten the background image (unless that's what you want).
 

Figure 3 - Bryce Scene with 2D Image Element Added (with Terragen Image as the Texture)

I usually set this image WAY off in the distance, usually 10,000 units away, and maybe further so it will be affected by Bryce's haze effect (which works quite well). As its off in the distance, it will need to be scaled UP to fill the entire background, or you'll be able to see that its just an image on a billboard off in the distance.   Look at Figure 4.  Now doesn't that look good?  Nobody will ever know it was a 2D image pushed to the back of the scene! 

Figure 4 - I pushed the 2D image back, scaled it up to fill the screen, and tweaked the haze settings to make the object "sit in" the image.  Think about the flexibility this will give.  A lot of movies use this same method.

Now, you can start to add mid-ground and foreground elements. These can be previously generated images on 2D billboards or 3D objects (or even pre-saved Bryce scenes you can merge). Be sure to add light spheres positioned appropriately for best lighting of these elements.  Remember, all light is turned off on this scene so you'll need something to light the foreground and mid-ground elements (notice how the ground in Figure 4 is quite dark).

Here's a note on 2D images used as billboards:
Billboards work best off in the distance (the further away from the camera, the better). If they must be close, they need to be very small.  Otherwise, you will get too flat of a look, and the fact that its a billboard 'image' will be obvious. So try to use true 3D objects for anything up close. Anything past the mid-ground of the image can usually be a 2D billboard.  And this will save time, system resources, and rendering speed if you make this the case.  You will also need to tweak the ambience for each 'billboard' to the best value to make the image fit in the scene. 

As for the background billboard, the ambience is kept at 100% to allow it to keep its true colors and brilliance. It can  purposely be set lower, and light spheres used up close to it to bring it back to brilliance. When done properly, this can add seriously sweet depth to the background image! It just has to be done properly, or will look quite odd.

And that's how I do most of my backgrounds!

 

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