Images embedded within the XHTML document as a base64 encoded string needs to specify a MIME type with content corresponding to the MIME specified. If a MIME type isn't specified, you will receive the error message Violation: “incorrectMIMETypeSpecified”.
This is due to the fact that ESMA's ESEF manual guidance 2.5.1 states that images embedded in the XHTML document as a base64 encoded string MUST have the correct MIME type specified.
Images Not Embedded But Referenced in the XHTML
ESMA's Manual gives the option to place larger images within the ESEF package and then referencing them in the XHTML instead of embedding them as base64 encoded strings. In this instance a MIME type may not be specified, in which case such files should match their file extensions.