@droopy: That's the settings yeah (i.e., on your screenshot, it's left to Nickel, calibre won't generate nor upload anything).
The Kobo Utilities plugin can re-process & re-send covers, but you'll want the patched version I linked to earlier, as the current release is missing some of the new options, which might lead to weird results.
FWIW, my own recommendation is to basically tick everything in there (i.e., Upload, B&W, Dither, PNG, Keep AR, Letterbox), but that's partly my personal preferences, so YMMV

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