You can use free synthetic humans in non-commercial projects only.
Images in half-size resolution (500x500) are available for personal use with a backlink to generated.photos. You can do as follows:
place the link near the image;
place the link at the bottom of the page where the image is used (website footer);
embed the link into the image;
add the attribution to the description of your project, app, or article.
Here's a sample of the attribution line:
Photo by Generated Photos
If your project is commercial, please contact us and share a bit more details about it, we'll see what we can offer.