I did search for the main image after my first designer died. I bought a license; I think it was $12. I found the other images she used in DepositPhotos, where like most people I already had a deal, and I grabbed them, too. Since then, another designer has made many modifications to the cover.
I presume that the designer's heirs--if they didn't just throw out her computer--might want to claim creator rights on the concept, except that I was the one who told the designer what the concept was to be, so it was work for hire.
Some designers might argue with that. In my long experience with work for hire, including overseeing contracts for such work, commissioning somebody to do what you tell them to do in a specified parameter but using their own tools and creativity in the details definitely qualifies as work for hire. There are other markers of work for hire, but yada yada. I am well aware that some designers like to hold onto the original files of the covers they create and act as if they own them and only licensed them to us. But in fact, that is not the core of the typical contractual agreement for a cover design.