Your method is a right mess. Here's how to do it so much easier.
- Copy the cover you want to the clipboard
- Load the KF8 (AZW3) file into the editor
- double click on the cover image in the list of files on the right side
- Paste the new cover over the existing cover
- Find the cover thumbnail and delete it
- Save the eBook and exit the editor
You are done and you didn't have to do any converting