You do have recourse available, but registering the copyright is a required step in seeking that recourse, and must be done before suing.

> Title 17 U. S. C. §411(a) states that “no civil action for infringement of the copyright in any United States work shall be instituted until ... registration of the copyright claim has been made in accordance with this title.”

> ... however, a copyright owner can recover for infringement that occurred both before and after registration

https://www.supremecourt.gov/opinions/18pdf/17-571_e29f.pdf

It's possibly an interesting wrinkle given that, especially for certain types of works, the vast bulk of material is likely copyrighted but not registered.

> interesting wrinkle

Far more then that, it's an absolute show-stopper for any potential lawsuits, as it's really unlikely any potential litigant will be able to prove that even a large majority of the images have been registered, and thus eligible for a lawsuit.

All they have to do is register their works that they are claiming were specifically infringed, and they are fine.

Well, with the registration issue. They still have to prove us of an exclusive right and overcome any argument that the other side puts up that the use involved fell under fair use, but they'll have dealt with the threshold issue of “are we allowed to sue over the works we are alleging were used".

> All they have to do is register their works that they are claiming were specifically infringed, and they are fine.

Based on what legal reasoning?