Uploading image to a node via Angular to Drupal 8 using REST
ImageItem and FileItem are represented as entity references. The ImageItem has the following schema: target_id, alt, title, width, and height.

Only the target_id column is a required property, which can be seen through ImageItem::propertyDefinitions > FileItem::propertyDefinitions >  FieldReferenceItem::propertyDefinitions parent method.

The field should be represented similar to the type property, which also is an entity reference.