diff --git a/core/nwb.base.yaml b/core/nwb.base.yaml
index a644c8d0..91c8abf8 100644
--- a/core/nwb.base.yaml
+++ b/core/nwb.base.yaml
@@ -254,4 +254,6 @@ groups:
       target_type: Image
       reftype: object
     doc: Ordered dataset of references to Image objects stored in the parent group.
+      Each Image object in the Images group should be stored once and only once, so
+      the dataset should have the same length as the number of images.
     quantity: '?'
diff --git a/docs/format/source/format_release_notes.rst b/docs/format/source/format_release_notes.rst
index 7c4f1310..3ce073b0 100644
--- a/docs/format/source/format_release_notes.rst
+++ b/docs/format/source/format_release_notes.rst
@@ -16,7 +16,7 @@ Minor changes
 - Allowed ``NWBFile/stimulus/templates`` to contain ``Images`` objects. (#459)
 - Added new optional "order_of_images" dataset to ``Images`` that contains an ordered list of object references to
   ``Image`` objects stored in the same ``Images`` object. This dataset must be used if the images are referred to
-  by index, e.g., from an ``IndexSeries`` object. (#459, #518)
+  by index, e.g., from an ``IndexSeries`` object. (#459, #518, #519)
 - Overhauled ``IndexSeries`` type (#459):
   - Fixed dtype of ``data`` dataset of ``IndexSeries`` (int32 -> uint32).
   - Updated ``unit`` attribute of ``data`` to have fixed value "N/A".