Not everyone fully understands when giving credit to someone's work is appropriate or necessary.
The answer: Always.
Whether it is a photo, idea, video, quote: It is ALWAYS appropriate.
General rule: If you didn't make it, think of it, create it, shoot it, make it or say it, you should give credit.
So HOW do you credit?
I found this awesome Poster on Design Sponge, (a great blog I just recently discovered.)
This poster is Helpful, practical, and of course... super pretty.