Before the new calibre icon theme (not calibre 3.x, but somewhere around October last year) the blank cover placeholder image was an image of a brown book, now it is a blue image of an "empty cover". In both cases there is no image associated with it. If you dont like the new image, you can always change it to what you want as described in the manual
https://manual.calibre-ebook.com/cus...ates-et-cetera