I've been playing around with these a little and the idea is that it is like an opened dollhouse with limited number of walls. Most are not designed for anything beyond the screenshot... ie not necessarily livable. Think of it as a cutaway view/diorama of a moment in a sim's life :) I also pull out the ceiling to stop it throwing a shadow and then in live mode play around with times of day and maybe even seasons to see what lighting looks best - though I don't use ceiling lights as they kept getting into the shot (I let the outside lighting 'pretend' to be ceiling lights :D ). It's not exactly a 'challenge'... rather it seems to be more around creating a picture/artstyle.
These are my first 2 efforts:
Granny's BedroomGranny's Kitchen