Many devs find it useful to inspect system artwork -- whether for inspiration, for reverse engineering to help create consistent new elements or simply for curiosity. In that goal, there's been no better tool than the open-source iOS Artwork Extractor from Cédric Luthi.
The project, formerly UIKit Artwork Extractor, has now been updated for pretty much any iOS version you throw at it, including iOS 7. It also now enables you to scan arbitrary frameworks, not just UIKit.
It's a handy utility, one well worth checking out.