Most photographer libraries contain images that cannot be replaced. An example is the sunset photograph that is being so heavily infringed. It can never be shot again. The beach area has been removed, the tree taken down. That area will never be available for shots like that again. I have shots in my library that have people and things on it that cannot ever be replaced. I have so many hours of work in it scanning negatives and slides since a lot of my work was done in that medium. It took years to build and organize it. I originally started the digital organization in 2006. The sunset is just an example of how one photo can go from being used on one site to becoming a much used commercial image. It shows how you must keep the records, have things organized to a point anyway and do not get rid of photographic images.
That is why I have said many times that you have to keep things on a business level and not personal. You never know when one image will take off and generate income. Not having them or the HR version of them is a killer to your income and your ability to prove you can deliver the work. I have also said I care for some of the infringers but they cannot be dealt with on that level. By taking the library or images and using them for profit and then complaining they did not know shows how little they cared.
I have a couple of shots from Honeymoon Island being used commercially without permission. One has all but been removed and paid for but the sunset with the palms is still growing and people do not want to pay for it because some infringers put it out saying it was in the public domain. It is another clear example of people not taking responsibility for their actions. They just like to blame someone else or use the excuse they did not know. I understand that people do not know things, it does not excuse them but when they are truly sorry they did something they fix it. If not then about the only thing they can be sorry about is that they got caught.
As a photographer you have options on many things. You will be infringed someday if you do good work. People like to get things for free and some will take them if they can. You have to make the decision on each one if you pursue it or let it go. To me, the person who did the infringing sets that standard by their actions or lack of actions. You have to make up your own mind, I have never and will never tell anyone how to make a decision like that.
Disclaimer: I am not an attorney. These are my personal opinions and general information only. This is not legal advice and should not be used as such.