How dare you post that picture of me without permission!
Jokes aside, I think this is a tough one to judge. If someone posts your content, say a video, on steemit and makes a few bucks off of it while gaining you some fans in the process it's a win-win. You'd probably have gladly sacrificed the content for a larger following, and they did the work to get it into people's hands. Basically they worked for you. But that doesn't mean you wanted them to, and it doesn't mean they won't keep stealing content for their own uses and trying to take credit.
I think as long as the content is shown to clearly be someone else's, and you've either gotten permission to use it or had no personal gain from it its ok. Sourcing would be the best thing, but sometimes, like for images, we don't have good sourcing available.
It's also easy to steal content by simply rearranging it or re-writing it in different words. Then the content is stolen, but how can you really tell? All they did was steal the concept. I could easily write a post about this same topic, and if I had a bigger following, more SP, higher rep, my article would outshine yours on this platform even though I couldn't put it as eloquently.
It's hard to judge, I know I wouldn't want content stolen so I would probably just keep an eye out and hope other people could alert me when they see it happen.