All books have two covers: one that is embedded in the book and one that is uploaded separately.
The book cover you upload separately is the one the stores use to display your book in the store in good quality.
The embedded book cover is the one that is 'inside' your book, in the epub file: if someone buys the book, that book cover comes with it and will be displayed on the virtual book shelf of the reader. Because most people read books on a smartphone or ereader, there is a maximum size limit many book stores (like Apple iBookstore) set: 4 megapixels (otherwise books would occupy too much space at the user's device).
Please resize the embedded cover using an external image editor (eg. PhotoShop or Paint) and reupload the file.
You can follow this guide for more steps: https://publishdrive.zendesk.com/hc/en-us/articles/115003167174-Ebook-formatting-too-large-cover-in-epub-file