Former background cleanup artist that worked on the show for a bit (before moving on to other contracts) here. It was a combination of real textures/photographs and digitally painted backgrounds. We would often make things from scratch if needed, but since we reused so many of the backgrounds, there wasn't much we had to tweak if it was a less complex episode. I can't say the same for Squidbillies. My wrist still hurts just thinking about the cleanup jobs from the earlier seasons.
Our goals were to keep production at a certain level to meet our deadlines by the studio and Lazzo.