The cover is added in the online conversion process. You should NOT include a cover in the book, else you run the risk of having two of them (one full page, one with margins like any interior image) especially if you upload an epub to the KDP. The software seems always to suppress the first image in a book file otherwise, if it's close to the front.
This has been the case since last fall, when we had the option of including the "product image" or not. Then the option was eliminated, and the product image (separately uploaded) always becomes the included cover. It is however considerably compressed.