@
stumped -
theducks has beaten me to it, try the
Generate Cover plugin. This is the sort of thing you can do with it, using my Billy Tea image.
Another way to get the image out of the epub you have, would be to right click it in the editor's files browser and use the Export image option, I suggest you put it on the desktop and then drop it onto the book details panel. That way you'll have a copy of the original at hand while you're exploring the options in the Generate Cover PI - you'll probably want to try different combinations of colours, fonts, border widths, positions etc.
Once you've generated a cover you like, you'll need to get it into the epub, I would use the Polish Book tool.
BR