The first day was that beautiful sky, but the pig escaped. The rope broke and it drifted off, up into the flight path at Heathrow. Then the next day, we flew another pig, and it was a bright blue sky, and so the photographs weren't nearly as interesting as they had been from the day before. So in fact we stripped the pig from the second day into the photograph from the first day that didn't have a pig in it because it had already escaped. And that is what appeared on the album cover.
The pig has been escaping ever since. The latest incident was at the 2008 Coachella festival. Link -via Metafilter