@killo3937 - As I only speak english I have no idea what Sigil is trying to warn you about. I will say the base functionality of this plugin has not been changed in a very long time. There is nothing I can do with that without (a) having a copy of your epub that has the jacket stilll in it and (b) knowing exactly which version of Sigil you tested it with.
So long as the epub displays in calibre viewer/still converts ok to other formats if that is what you use in your device then I simply would not worry about what that Sigil check is telling you. Things like EpubCheck can generate a lot of "noise" about things which simply do not actually matter - just chill and go read the book
