"Rare images" and "rare footage" are common phrases in linkbait. It's a particularly ham-fisted attempt to sensationalise a topic. The idea fails to make sense at a very basic level. An image can be duplicated. Scarcity doesn't apply. So images can't be rare. In time to come we'll see awkward attempts to redefine the meaning of 'rare' so as to contain this awful usage.
You can't have ever had a difficult time finding a particular image, or sometimes any image of a particular thing before. In the case of footage, I've spent years trying to find footage that I knew existed at some point (based on somebody's writing, or from stills in a book), but short of getting access to some archive and flying (internationally) there to search through boxes, I not only can't find it, but can't even verify that it still exists.
Just because a thing can be duplicated doesn't mean it has been, and just because something was once available doesn't mean you'll have any luck finding it today.
