PSD

From GDWiki

PSD is the native file format of Photoshop. It is a lossless file format, which saves a great amount of information used by the program; information such as alpha channels, paths, texts, masks, color format, layers -- the list continues. The drawback of having much information is obviously the file size, which means that PSD files are generally not suited for games. They can however prove to be valuable in the creation of game graphics.