It used to be CC0 on Unsplash but some time ago they moved from that to a custom license with some special restrictons, which makes it impossible to use in other Creative Commons work: https://unsplash.com/license
This also happened to a lot of other similar photo sites unfortunately.
Preferred for the feed would be RSS I’d say, but maybe Twitter is good on top for promo.
The WTF license is an antipattern really. All it does is push away anyone who has serious use cases for your work, because of the risk that nothing in that license will (likely) hold up in court. MIT carries basically the exact same provisions as WTF, so just use that.
But, no. The licensing will be up to the artist, always.
What would be the preferred way? Twitter? RSS?