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pslsr - the Calibre Library Manager has a couple of cover generation tools, a built-in one that you can access from the Metadata Editor, and an optional plugin - Generate Cover - see ==>>
Index of plugins.
After creating cover from one of the above you'll need to embed it in the epub file, the easiest way to do that is with the Polish Book tool, select the book and press 'P'.
Or, if you're using the book editor in a stand-alone mode - i.e. without a library, export the cover.jpeg (right click the file), edit it with an image editor, and then replace cover.jpeg (right click the file) with the edited version.
Or use Sigil's Open With to do the same thing… more or less.
BR