I can't speak to tattoos specifically, but I'm familiar with IP contracts in other creative areas, and in every one of those cases, no contract means the copyright belongs to the creator.
For the US, if the creator did the work without initial payment or the promise of payment on delivery then the copyright is theirs. If they are hired before it exists, it is work for hire.
Does anyone "hire" a tattoo artist to do their tattoos? Other than a tattoo parlor?
For this to be true, "hire" does not just mean pay someone for a service, it has to mean that that person has become an employee of your company.
For example if you "hire" (contract with) a photographer to shoot your event, the photographer retains copyright on those photos unless you negotiate work-for-hire and document it in the contract. But if you have a company with a photographer on staff, the employment contract would typically specify that the company owns the copyright to images taken in the course of work.
