Function lower_image_directive 

pub(crate) fn lower_image_directive(
    document: &mut Document,
    root: NodeId,
    args: &[SetArg],
    span: &SourceSpan,
    inputs: &mut ExternalInputs<'_>,
    em_pt: f64,
    diagnostics: &mut Vec<Diagnostic>,
)

Lower #image(...) into one [NodeKind::Image] node.

Decoded pixels and dimensions are stored in node attributes so later stages do not re-open the source file.