As far as I am aware (correct me if I am wrong) there are no plug-ins to edit the Book Cover.
I was thinking that such would be good - or even better - for it to be a built in feature.
So for example - let's say I have a "dirty" cover that I want to "clean up" (graphically). The option would be there to EDIT the cover image - either directly within Calibre (if such tools were included ... similar to tools to edit epubs - but for graphics) or to have the cover opened in an app/program that can edit the image file.
So again for example - just to be clear - let's say my cover has an unwanted black bar covering part of the cover (say from a bad scan). And I want to replace the black with the color of the actual cover (let's say red). In the editor I can select the black area then fill it in - using a color picker to match the cover's red - with red. It would then save the image file back to the book's directory - which now would have a "cleaned up" cover.
Another good example - You could reorient the cover - let's say it is slightly off the vertical. You could ROTATE the image to align it properly. I think this would come in handy.
Just a thought and suggestion. Would be interested in other people's thoughts.