The game used some CC0 pictures, which are from the following platforms:
Pixabay, Check the license information:
ambientCG, Check the license information:
Poly Haven, Check the license information:

Here's an excerpt from the CC0 FAQ page to clarify:
Once the creator or a subsequent owner of a work applies CC0 to a work, the work is no longer his or hers in any meaningful sense under copyright law. Anyone can then use the work in any way and for any purpose, including commercial purposes. Think of CC0 as the "no rights reserved" option. CC0 is a useful tool for clarifying that you do not claim copyright in a work anywhere in the world.

More info:
CC0 summary:
CC0 wiki:
CC0 FAQ:

Cat Head Games respects the rights of creators. If you believe that your intellectual property rights have been infringed by picture content from the above websites and platforms, please email us and include the following information:
1, Identification of the intellectual property right you believe has been infringed.
2, Identification of the content that you believe is infringing your rights, including in-game screenshots and videos showing when and where the content appears in the game.
3, Your contact information, such as your email address.
4, A statement that you have a good faith belief that use of the content in the manner complained of is not authorized by the intellectual property rights owner, its agent, or the law.
5, A declaration that the above information is accurate and that you are (or are authorized to act on behalf of) the intellectual property rights owner.
Nothing above constitutes legal advice. We suggest you consult an attorney regarding your rights and obligations under applicable laws.
We will remove images that infringe your rights as soon as possible after verifying the situation.