Bryce backgrounds are pixelated when they are enlarged. I noticed that when I make the bryce back ground exactly the same size as the window I am working on they look great, but even if render to a larger window the background will tend to pixelate. Probably the only way to avoid this is to put the background on a square which will pixelate it if the square is manipualted too large, or simply make the bryce background a larger size. I have tried different things but this is a function of the poser "background" feature I am guessing.