Manga Studio
From Just Solve the File Format Problem
Manga Studio (also known as Comic Studio and Clip Studio Paint) is a program used by artists in the creation of electronic comics. Its highest-priced version creates stories which are composed of pages (the lower-priced version only creates single pages not organized into stories); a story is saved in a .cst file, while a page saved separately from a story is saved with the .cpg extension.
This software originated in Japan where it was designed for manga-style comics, but current versions have features useful for American-style comics as well.