First of all, thanks for this useful plugin!
If I add a cover using the Calibre Editor tool and then I apply Editing Toolbag to any tag, epub-check finds an error on the titlepage (the one with the cover):
Code:
ERROR RSC-005 /text/titlepage.xhtml
(line 16, col 53) Error while parsing file 'attribute "preserveaspectratio" not allowed here; expected attribute "alignment-baseline", [...].
I can't really see the difference, but some modification to the file is applied.