An ePub book can have a picture as cover or not.

If you already have a cover page in your HTML files or in your Word document, most readers will take it(the first page) as the cover.

But some devices still need you assign a cover picture.


The cover picture can be any of the following format:

To fit for various stretched size and screen scale and resolution, JPEG format is recommended.

Resolution and scale

Cover picture can be of any size. But it's better fitting the screen size of target device.
Most eBook reader's screen scale is 3:4, and 600*800 is a popular screen resolution. So a 600*800 or 300*400 picture is recommended.

File Size

Please do not use big image file, which will make your book file very fat and slow. On the same visual quality, smaller(file size) is better. This rule is also fit for images inside your book.