I think juju's point is important especially if the "quoted" material includes opinion or editoral, and I do try to preface such remarks with a reference, for the good reasons he stated.
But it's probably not necessary in every case, such as this post, which is a caption to a pic and the credit is given at the end as is standard in the media itself.
And it wasn't one of those posts that you have to scroll to get to the bottom.
Anyway, juju, point taken.