No images inside the ebook can be larger than 4 megapixels. 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 2,000 pixels tall and 2,000 pixels wide, that is 4 million pixels (2,000 x 2,000).
The same is true for the embedded cover as well. The embedded cover is the one that is "inside" your book, in the epub file. If someone buys the book, the embedded cover will be displayed on the virtual bookshelf of the reader. Because most people read books on a smartphone or ereader, there is a maximum size limit many bookstores (like iBooks) set: 4 megapixels (otherwise books would occupy too much space at the user's device).
Check the dimensions of your cover to calculate the size in megapixels, then follow these steps to resize your image or cover image.
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 using Sigil or Calibre. Don't forget to delete your original picture from the book. If you are resizing your embedded cover, don't forget to mark it as cover again.