Images from movies: Production company, which can be found on IMDb under "company credits".
Images from TV: Production company and/or distributor (the network).
Photoshoots: The photographer's name if available, and the name of the magazine or site it's for.
Icons, fan art, or anything that's been edited by a fan: credit the person who edited it. (username @ site) Crediting the actual owner (network/production company) is a good idea here too.
Helpful articles:
link by DrDevience
link by misanthrope86
