I would appreciate it if anyone who writes something here would read what has already been written before doing so.
First of all, I think it's pretty clear that I don't intend to publish anything without first seeking the appropriate authorization from the owners of the photos.
Secondly, for those who don't know, there is a provision in all copyright regulations called "obras huérfanas" in Spanish, regulated in Article 37 of Royal Legislative Decree 1/1996. This provision refers to works for which, after diligent action, it has not been possible to contact the author and establishes a series of conditions such that, if met, their use is legal even without prior authorization.
In the United States, the regulations merely acknowledge the problem but offer no solution, limiting the proposals to limiting the liability of those who use them after a diligent search for the author. An example is the Google Books case (Authors Guild v. Google, Inc.).
In the United Kingdom, the applicable regulation is the EU Orphan Works Directive, implemented by the Copyright and Rights in Performances (Certain Permitted Uses of Orphan Works) Regulations 2014 (SI 2014/2863), similar to the Spanish model.