“Yet the image is still protected by copyright.”
So? That image is someone else’s property. At what point should any property become public domain? After 5 years can I simply move into your house?
You are correct about that but what about abandoned property? Most books for example are never republished after a very short run. It was proposed a few years ago that copyrights holders be required to protect their copyright by filing paperwork every few years showing they wanted to protect it.
Quite a lot of textbook and other works could be in the public demand as a result. The copyright holders no longer have a commercial interest in the work but the material is unavailable none the less.