Gallery Picker Plugin
The Gallery Picker plugin provides an "imagelink" field type that can be added to any document type's editing template. This allows authors to select an image from the gallery and include it in a document.
The Gallery Picker plugin is part of Hippo's default configuration (when creating a project using the archetype).
The imagelink field type is listed as a Compound Type in the Document Type Editor's list of available field types:
Image preview in a document:
Image Picker dialog: