Let me just clarify that I'm not a lawyer or even an expert on copyright. If you plan on making a business out of this you should do some research and find some more reliable sources than me. ^^
Some quick googling turned this up, which seems to answer a lot of your questions.
At it's core copyright is simple: If you're the original creator of something it's yours to do with as you please. If someone else is the original creator you have basically no right to do anything with it at all, unless the creator has explicitly given you permission.
As the term "copyright" implies, you're technically not even allowed to "copy". On traditional media that was easy enough to enforce. The internet has turned that upside-down, because when you view a picture on your own device you've already technically copied it into your device's memory. That's why copyright has turned into a massive grey zone of mystery and confusion.
Things get complicated when more people are involved, especially when they come from different countries (laws, jurisdictions). I'd go as far as to say that the only way of really being safe is to just always create original work. If you plan on selling things that's the route I'd take.
In reality the vast majority of copyright infringements are so minor that simply no one cares. That doesn't make them legal, it's just very difficult/expensive to prevent it. That's why as long as nobody is earning any money publishers tend to ignore it or even support it. What would a publisher gain from expensively tracking down thousands of people to prevent them from posting essentially free advertisement?
Of course once real money is involved that may change very quickly. Even more so when you deface things in a way that makes them offensive. For example if you start plastering Mickey Mouse dressed as Hitler all over the internet you might get an angry letter from Disney.
As for the average hobbyist/freelance artist on the internet, they typically don't even have the resources to really prevent others from copying their work. They can yell at you and maybe if they're from the same country they may be able to take some legal action, but that's rare enough.
In practice I think it's really more of an ethical decision rather than a legal consideration. If you copy an individual's work it's only fair to let them know (or better ask beforehand). If you make money off of someone else's work, don't you think they deserve a cut?
So with these considerations in mind I'll try to answer your questions:
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The guy who created the picture holds it's copyright, so he's the one to talk to. If the picture shows a character that is owned (tradmarked) by someone else that's mostly between them. Of course the reality is that they probably never talked to each other to begin with so I wouldn't worry too much about it.
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Probably both.
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Ask them. Check their website. In reality they probably probably benefit from fan art and don't have a way to prevent it anyway. But that doesn't mean they necessarily accept it.
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"Royalties" means money. Typically if you sell artwork based on characters owned by others, they will allow you to do so on the premise that they get a share of your earnings.
sorry im dumb lol.
Nope, this shit is just confusing. I hope I could help a little.
