Its not quite as complicated as it seems, it essentially boils down to this.

1 - If someone is paying you to do something, the two of you have entered into an agreement, that agreement is considered a contract in most legal jurisdictions (there are some contracts, in certain jurisdictions which require a contract to be written to be enforceable, but in most jurisdictions even a verbal contract is binding).

2 - A traditional contract regarding the creation of something (music, art, photography, etc) includes a clause which dictates to whom the various rights to the finish product belong. This often includes rights to alter, edit, repost, and even resell that piece of work in perpetuity.

3 - Where this becomes more complicated is when a contract doesn't exist that lists those rights. In the event that a contract doesn't stipulate which rights belong to which party, either side may dispute the contract legally and a court will adjudicate. This means that the court will decide what the agreement was, and their decision is legally binding. Both sides would appear before a judge and tell the judge what they believed the agreement was, the judge will then consider each side's arguement and make a binding decision about the work.

(Now, lets illustrate an example. I approach someone in the fandom and I pay $500 for a very nice black and white logo that I plan on using as signage for my company "Wolfstar Cafe". I plan on having this on the sign at the front of my cafe and having it printed on all of the disposable mugs that I hand out to my customers. Lets assume that I'm new to business and copyright law and I don't get a written contract releasing me to use this art in that matter. The original artist sees their logo being used and they flip their shit and demand that I stop using the logo because I didn't ask them for permission to use "Their Art" for my business. I return their message and say "What are you talking about, I commissioned this piece BECAUSE I needed a logo for my business". The artist decides that this is ridiculous and they chose to sue me for copyright... I appear before the judge and say "Your honor, I told the artist that I was starting a cafe and needed a logo... while I didn't expressly tell them HOW I was going to be using the logo and what it was going to be printed on, it was obvious based on our conversation that this was the intended use of the piece. And why would someone pay $500 for a black and white logo of a wolf's head if they weren't interested in doing anything with it". The judge obviously hears the opposite from the artist, and then they make a binding ruling based on the case. Its very possible that the judge will rule against me and say "Because this wasn't discussed in detail, you may not use it for this purpose" but it's JUST as likely that they will turn to the artist and say "If you want to withhold specific rights regarding a piece of commissioned work, you need to write a commission agreement that stipulates which (if any) rights you wish to reserve... they can do whatever they want with the logo, they paid you for it")