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.