According to ESMA's ESEF Manual guidance 2.5.1 any image included in the XHTML document should be a base64 encoded string. If an image in the file isn't a base64 encoded string, you will get the error message Violation: “embeddedImageNotUsingBase64Encoding”.
Note: We as well as ESMA recommend that you do not embed images with financial information in the iXBRL document. Images should solely be used for branding information, graphical layout, photographs, etc.
Including Bigger Images
Different browsers will have different limits on the size of base64 strings. In case the size of an image exceeds the support of browsers, they can be contained as separate files within the ESEF package.
What is Base64 Encoding?
Base64 is a group of binary-to-text encoding schemes which is designed to carry data stored in binary formats between channels that only support text content. Base64 is most widely used on the world wide web, and its functionalities include the ability to embed different binary assets (such as images) within HTML and CSS files.