According to the directory’s Frequently Asked Questions, it
…provides free, high-quality, CC0-licensed, community-submitted photographs suitable for use within WordPress sites and elsewhere.
Which means, photos in the directory can be used without attribution or payment.
A great resource for anyone looking to use photos, personally or commercially.
Be aware copyright remains with the photographer who took the photo.
When I heard Mullenweg speak about the WordPress Photos Directory, I understood it wasn’t live.
But I was wrong, the directory has launched.
Though the message on the site says it’s not fully launched, you can download different versions of photos in the directory.
In addition, you can submit your photos to the directory.
At this time, you can only submit one photo at a time.