This is what the license states:
"It's hard to understand complex licenses that is why all photos on "name of site" are licensed under the Creative Commons Zero (CC0) license. This means the pictures are completely free to be used for any legal purpose.
- The pictures are free for personal and even for commercial use.
- You can modify, copy and distribute the photos.
- All without asking for permission or setting a link to the source. So that attribution is not required."