I don't know how/where K4PC get its covers when K4PC shows a different image than the one that's part of the mobi. K4PC may be downloading an image based on the ASIN number for all I know.
Mobi's may have a
thumbnail cover image in addition to the
cover image:
1) The four bytes contained in EXTH Record Type 201 are added to the first image field (offset 108 of the mobi header) to find the cover image's PDB record.
2) The four bytes contained in EXTH Record Type 202 are added to the first image field (offset 108 of the mobi header) to find the thumbnail cover image's PDB record.
* But you probably know all that already since you referenced the Mobi Format Wiki here on MR.