Ah sorry. I did read the text, my English is not so good.
So water marks are cool until you get a contract ready for the game creator? You should be able to study the sprites for nudity or whatever with subtle watermarks.
Hope I dont come across as too paranoid. I have been outright ripped off for my artwork on more than one occasion.
My Protocol now is to Watermark everything. Add contact information to EXIF METADATA information area, along with copyright information.
Copyright information should be displayed at all times, and verbosely name the creator with lots of contact information. Is what my lawyer said.
Also on the image area, there should be some sort of visible Copyright information.
The game run time should have the copyright information clearly displayed at all times.
People without a lawyer can have art and music protected with the Creative Commons license if it is set up properly.
https://creativecommons.org/choose/
Everyone! even kids can protect their content. They even have legal teams to fight in court for you!! Only takes a few clicks and your dream game is protected!
If you are Legit, then a Legit contract should not be too much of a problem to produce before you get the game delivered to you. Then there are no questions later, and everyone is protected to a certain degree.
If we agree to your terms of service, then you should also have to agree to our terms as well. On paper, in a contract.
I hope you are just new at this and it is an over site. The service you are offering seems really nice.
If it all works out, I think it could work well for everyone involved.