Quote:
Originally Posted by nando-debug
The problem is that through GUI I can do this, but not through the command line.
This doesn't work for any conversion I've tried, even converting to .azw3 format, but it doesn't change.

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Since you are converting from ePub, use the GUI with the Modify ePub plugin or one of the Polish options. This will change the cover in the ePub and then you can convert to KF8 from there for your Paperwhite.
I've never tried to replace the cover from the command line. I don't know if it can be done. But I see no problem doing this from the GUI.