The "Simple Section" document type has an image element. The source of the image is specified using required attribute source, which contains an absolute or relative URI. An image element also has optional width and height attributes, which contain dimensions expressed in pixels.

This image element is styled using replaced content. Excerpts from rng_section_config/section.css (same file for all variants):

image {
    display: inline;
    content: image-viewport(attribute, source,
                            data-type, anyURI,
                            content-width, attr(width),
                            content-height, attr(height));
}

This replaced content consists in extension pseudo-function image-viewport() in XMLmind XML Editor - Support of Cascading Style Sheets (W3C CSS) or extension pseudo-function image() in XMLmind XML Editor - Support of Cascading Style Sheets (W3C CSS). Note that image() is based on image-viewport(), except that it has a simpler syntax. For example, one could have styled the image element as follows:

image {
    display: inline;
    content: image(attr(source), attr(width), attr(height));
}