In guidance 2.5.1 ESMA's ESEF Reporting Manual states that images embedded in the XHTML document as a base64 encoded string MUST have the correct MIME type specified. If no MIME type is specified, you will get the error message Violation: "MIMETypeNotSpecified".
Images not embedded but Instead referenced in the xHTML
ESMA's Reporting 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.