Originally Posted by Fluribus
I would do it if I had a one-step way of inserting the cover into the existing file rather than running everything through a conversion process.
I've got a few Hachette books that don't have proper covers either.
You do have a way of a doing it if you have an AZW3 version of the eBook. I know you have a Kindle and there is no way of doing this with a Mobipocket eBook. But with the AZW3 version, you can use Tweak eBook in Calibre to replace the generic cover with a cover image of your choice.
ePub is even easier as all you do is edit the eBbok's metadata, replace the generic cover and use the Modify_ePub plugin to replace the cover. That does it without converting the ePub.