Copyright Laws are there to protect the rights of person who created the original work, as well as to act as guidelines for persons wishing to use other's work.
That being said, observing the letter of the Law is nigh on impossible.
Where there is an obvious and flagrant attempt to pass another's work off as your own, or to avoid paying for the right to use it, then the matter is cut and dried.
But when the originator is difficult or impossible to trace for one reason or another then common sense tends to prevail.
If the original copyright owner, or a descendent, should appear to claim breach of Copyright they would first have to prove that they held Copyright and that the Copyright had not expired.
They would then have to prove that you had used the images to make a profit at their expense.
If you acted in good faith, believing the Copyright to have expired after making a reasonable effort to trace the copyright owner, then I don't think there would be much of a case to answer.
If you had made a huge amount of money from the image then it would be reasonable to pay some of this to the Copyright owner once they had established their claim. The amount could be set by the Court, or more likely negotiated as an out of Court settlement.
If you made little or no money out of it any settlement would probably be a token gesture. If it ever got to Court in the first place - Lawyers know that you can't get blood out of a stone.
The bottom line is: weigh up the chances of the original copyright owner catching you and making a fuss. If you don't like the odds then walk away.
But in this case the chances of being killed by a falling coconut are far greater.