I understand the OP confusion.
A book can only have a cover. At least paperbooks have (the others are named differently).
So if one marks an image as a cover, why the (X)HTM(L) file can be set as well? And if it's required to be this a cover why the image retains its status?
Speaking of this: I have yet to find the "uncover"
option,
id est to remove the cover-status of a file....
Anyway, no hardware eReaders I have was able to display an image as a cover, only XHTML files (which may include an image).