I've seen some stories on SteemIt that have been completely lifted, or stolen, from other sources. Copying and pasting whole paragraphs of stories without rewriting the content is blatant plagiarism and should be avoided. It's a good way to get flagged and lose steem reputation.
Inserting a link to the story is not good enough. At the least, a Steemian can use quotes to indicate content that is not their own. It's even better to rephrase or rewrite the content entirely. The best policy is to do some of your own research from multiple sources and craft an original story, like in Sushi, Parasites and Stomach Pain, Oh My!
This goes for photographs and images as well. Provide a "courtesy of" tag to the image to indicate that it's not an original piece.
Steemians should not be thieves, and passing off someone's work as your own is plain and simple intellectual theft. You may gain followers by regurgitating other people's work at a break-neck pace, but in the process you lose credibility.
Social media sites like FakeBook are filled with those kinds of folks. Let's keep the content on SteemIt original, and let's accurately and diligently cite sources when we borrow from the work of others.
For more information on avoiding plagiarism, check out 6 Ways to Avoid Plagiarism in Research Papers. The article is academic-based, but its contents apply to stories created for SteemIt.
Steem on, Steemians (but do it responsibly)!