A Press-Kit is what goes out to the different publications and sites, along with artbooks etc, and they normally contain game artwork in .psd and high res .jpg, system pics, logo's, screenshots of upcoming titles, some contain midi music and trailers to, some also contain other media such as calendars etc.
Include:
Animal Crossing
e-Reader
Kirby GBA
Mario Party 4
Metroid Fusion
Metroid Prime
Star Fox Adventures
Super Mario Sunshine
Wario World
Yoshi's Island- SMA3
Images are in tiff, psd and bmp format. And range from artwork to screenshots.