Art guidelines

You can use any assets that you have the right to do so.
e.g. any assets that have been distributed under the following licenses:

https://creativecommons.org/licenses/by/4.0/
https://creativecommons.org/licenses/by/3.0/
https://creativecommons.org/licenses/by-sa/4.0/
https://creativecommons.org/licenses/by-sa/3.0/
https://creativecommons.org/publicdomain/zero/1.0/
http://www.gnu.org/licenses/gpl-3.0.html
http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
https://opengameart.org/content/oga-by-30-faq

Or of course your own work for which you own the copyright.