I just read a bunch of questions and comments on the contest site. Digitally created images are allowed as well as use of non-LEGO objects like rocks and trees. At the very least, you need “crooks” and “LEGO city mountain police,” for example, this guy:
https://www.bricklink.com/v2/catalog/catalogitem.page?id=160992#T=S&O={“iconly”:0}