Each pack preview thumbnail link will lead you to Google Drive where you can download the ZIP file.
Note: some ZIP files are too big for Google to scan and show a preview of the contents, so don’t worry, they’re safe to download.
Here you can see large preview images of each pack before download.
Reference Photo Packs Terms of Use
If you want to download multiple packs at once, here’s a link to the entire Reference Photos Packs folder.