What does the law say?
This is a dog damn mess 💀🐶. First, any picture you publish is both dependent on the rules and licensing of Stack-Exchange and the site you take or publish it on. In the case of pictures this generally means you need to abide by Imgur's rules, since copy-pasting the picture directly uploads to it.
On top of that, you have to consider whether you have enough rights to said picture, which implies many other sub-topics. In France for instance, you have to get the written agreement of any non-public person to have their image published if they happen to appear on a photo... When they're not part of a crowd. And when they're clearly distinguishable. And you need the agreement of the photograph, too. And the agreement would be only valid for usage on WB:SE. In the U.S. there is at the very least the Fair Use, and I won't be surprised to see many other edge cases that can be as convenient as they are complicated.
But supposing you have all the rights outside Worldbuilding Stack-Exchange to publish it, you need to make proper attribution, as it is required by CC BY SA 4.0, the current license any content is posted here under. Missing attributions is a violation of the license.
What do I say?
Now that the theory is out, let's get into the real-world 🌎. As you noticed, I've made a whole speech about ownership before attributing it. First because that's necessary to understand why you need to do so, but also to make you understand that copyright, copyleft and copytheft are never easy topics to deal with. And contrary to what some of the Meta's meta posts say, plagiarism is strongly linked to copyright, even in the context of Stack-Exchange only. By taking content under CC-BY-SA 4.0 without attributing it, you breach this license's conditions of use. Worse, if you take content outside WB:SE from someone without their permission, this can lead to more serious troubles both for the user and Stack-Exchange in general.
If you are the one taking content from others
If you don't know enough about law, just keep yourself to your own content, or content either under open licenses or in the public domain (⚠️ : Being economically free does not always mean open license!). If there's nothing explicit about the license, assume you don't have the right.
As soon as you reuse content which is not yours, give at the very least a link to the source and its name. Ideally, add the author's name if it is known, and ultra-ideally add the license used1. It will not only make you look more amenable to the author and law in general2, it's an actual professional practice.
My way of doing it is to put a description right under the content I take from (
[space] twice +
[enter] once), then put a short description and the attribution. You can make it smaller by putting it between "
<sub></sub>" tags. Here's one I made for instance :
[![A drawing of the Catocala electa moth]] ---> 2 blanks here
<sup>One of my favorite Moths. Public domain, [from Wikimedia Commons]</sup>
This gives :
One of my favorite Moths. Public domain, from Wikimedia Commons
If you review content posted by another
I've already delved into the topic of ownership in another question, and bits of it applies quite well here :
But worse, noone could prove who catched one of them first.
Finding the true owner of some content is generally painful without any link to the source. Yes, there are search engines to backtrack pictures, people are often lazy and take the first result on the web and so on. But what makes you think user X on Devil-ant-art.com is not user Y on WB:SE? And how long did you take to find the source, when all that time could be spent elsewhere reviewing, answering and improving questions?
It is not our legal responsibility to make others respect it. It is the responsibility of the intellectual property owner and Stack-Exchange to take action when illegal stuff happens like breaching NDAs or infringing copyright.
As "basic" users of this site, we're only responsible of what we post, not the other's. Yes, it will be detrimental to our experience on this site if Stack-Exchange is sued for breaking copyrights, but we're not an employee of SE. You don't have to check that if you don't want 🐦. For moderators it's certainly more complex as usual, but a good look at your specific rules should help out. If not, you really need to clear out this fog ASAP.
You can (ideally should) warn the content owner of potential infringements so they can take appropriate measures -like contacting SE's designated agent- if they wish so. You can warn the SE company of possible troubles, so they can take preemptive measures if that's clear and needed. You can and should also keep people from raising obvious red flags for their own sake, both to learn worldbuilding and to be friendly. But closing and/or deleting in SE's stead? It's not our job, since it's impossible to know at all whether the user is the actual owner. We're just for most anonymous pals on the internet who can only reasonably argue they're who they are by telling their "proofs" are the truth. It's a very weak argument called circular reasoning.
And that part is basically what I advise. Any very obvious illegal content (watermarked picture, etc.) : Don't attribute but warn of potential copyright troubles to the user and Stack-Exchange. Dubious content? Ask for the source even if only for the sake of being professional and respectful. That's the gist for users who are not moderators. The ones who can answer the best and quickest where some content is coming from are the ones who reposted it and the original owner. Since we generally don't know the owner, we'll have to make up with the supposed good willingness of the one who reposted it.
For moderators, I am just not able to tell you your obligations. My intuition tells me you'll have to put down non-attributed content instead of kindly asking where it comes from. But please, whatever you have to do, don't do like other social websites. If you can avoid turning this attribution thing into a (mindless) copyright witch hunt to the detriment of everything else... You'll have my tanky thanks.
1 : Let's be honest here though, I doubt anyone will care if you forget to add the exact license. But if you think of adding it every time, it's great because it means you are sure you have the rights to it. If you happen to be wrong, you give proof you tried to be in the clear.
2 : Let's recall that a good chunk of legal issues are solved out of court. Any sign of kindness and honesty helps in that regard.