It’s hard to understand complex licenses that is why all products 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 products.
All without asking for permission or setting a link to the source. So, attribution is not required.
The only restriction is that identifiable people may not appear in a bad light or in a way that they may find offensive, unless they give their consent.
The CC0 license was released by the non-profit organization Creative Commons (CC). Get more information about Creative Commons images and the license on the official license page.