Um, you need to modify the actual book itself for that.
It has
nothing to do with the Kindle, for the record.
Nor importing (importing a book does not modify it).
...
It can be added during conversion,
OR
Use the "Polish books" tool (the default shortcut key is "P", or add it to your toolbar in Preferences ==> Toolbar )
One of the options is 'Add/Replace metadata as a "book jacket" page'.