Creating a linking image:
Cut a screenshot to 1280x814 (or 800x509 or 640x408 or other resolutions with the same aspect ratio), then scale it to 410x168.

Creating a book cover:
The image must be 512x512 pixel in size. The cover itself has a size of 127x291 pixel.
The top left corner of the cover is at pixel 18,106
The bottom right corner is at 145,397
Some books have one pixel more in one or two directions, so I'm not 100% sure about these values. You can use one of the other covers as template.