MOBI Validation Error: Cover is too small

Reduce amount of "Cover is too small" errors we see in the logs from kindlegen.

* Ideal height/width ration of 1.6
* A minimum of 625 pixels on the shortest side and 1000 pixels on the longest side
* For best quality, your image would be 1563 pixels on the shortest side and 2500 pixels on the longest side

@see: https://kdp.amazon.com/self-publishing/help?topicId=A2J0TRG6OPX0VM