If you receive the message Violation "imageFormatNotSupported" it means that an image within the xHTML document of your ESEF package is saved in a non-accepted format.
In guidance 2.5.1 "Inclusion of other content than XHTML and XBRL in the Inline
XBRL document" ESMA has stated that images included in the XHTML document MUST be saved in PNG, GIF, SVG or JPG/JPEG format.