David Blanchard Photography: Gallery contains thunderstorms, lightning, and tornadoes.
Historic NWS Catalog of Images: Several pictures of every type of cloud.
NOAA Photo Library - Beginnings and Endings: Photographs of sunrises and sunsets.
NSSL Photo Album: Photos of severe weather from the National Severe Storms Lab.
Weather Image Galleries: Contains images of mostly lightning.
Weather Photo Gallery: Contains pictures of mostly clouds and storms.
Weather Photography: A large collection of weather and atmospheric photography.
Weather Underground: Current Weather Related Photos: Contains weather and other outdoor photographs.
Weather.com - Photogallery: Pictures of storms, lightning, and travel shots. [May not work in all browsers.]