title_body: Cover image size by ebook format
upvoted_answer: This depends on where you are publishing your e-book, and is subject to change. With Kindle Direct Publishing, they require a minimum of 625 pixels on the shortest side and a minimum of 1000 pixels on the longest side, but they recommend 2500 pixels on the longest side. With Nook, they require that the height and width be at least 1400 pixels, and the file size cannot be greater than 2MB. I highly recommend creating your cover as a PNG file so that you can resize and save copies as JPG files so that they will be smaller in size.