No images embedded in the book can be larger than 4 megapixels: 4 million pixels. The total pixel size is calculated by multiplying the length and the width of a picture. For example, if you have an image that is 2000 pixels tall and 2000 pixels wide, that is 4 million pixels (2,000 x 2,000).
Check the dimensions of your cover to calculate the size in megapixels.
Fix it using Calibre
1. Open the book with Calibre Ebook Editor
2. On the left panel, select the image you'd like to resize.
3. Double-click the image to see current dimensions.
4. Click 'Resize'.
5. Enter your new dimensions.
6. Press 'Save': you're now able to upload your book.
Fix it using MS Paint
1. Open the image on your computer with Paint. If you no longer have the image on your computer, you can right-click your image in your ebook editor, and save it on your computer.
2. In Paint, click 'Resize', and select 'Pixels'.
3. Save your image, and add it to the book again. Don't forget to delete your original picture from the book.