In school it was made very, very clear how important it is to credit sources that are not yours. A quote? Credit it. A photo? Credit it. Even factual information that was used to inspire your own composition? Credit it. In college, you could get into a lot of trouble for not crediting someone else's work.
So I figured it was just common sense, something everyone knew. If you present work that is not yours, you credit the person who created it.
But more and more I have been seeing my own photos, and fellow bloggers photos floating around blogs, and especially Tumblr, completely uncredited.
I'm not sure if people just don't know it's what you're supposed to do, or if they just don't know how.
So if you're confused, here's a good way to credit someones work. Example:
That's it. "Disneyland" links to the post which it came from, "Vixen Vintage" links to the homepage.