Hrm. Well, what was explained to me when I did a challenge is that besides the main rules, I could make additional restrictions. It's up to the one making the challenge to determine how much postwork is to be allowed, whether to allow non-Bryce objects (imported stuff from like... Poser or something), etc. Overall, though, it really must be a Bryce render, not something like.. a 2D image rendered in Bryce... and the focus of the theme must be the subject matter. But the one who made the challenge really must be the one to clarify all this.