diff --git a/.gitmodules b/.gitmodules
index 80ce81d..9f34e0d 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -16,3 +16,6 @@
 [submodule "book/external/TeachBooks"]
 	path = book/external/TeachBooks
 	url = https://github.com/TeachBooks/TeachBooks.git
+[submodule "book/external/Sphinx-Image-Inverter"]
+	path = book/external/Sphinx-Image-Inverter
+	url = https://github.com/TeachBooks/Sphinx-Image-Inverter.git
diff --git a/book/_toc.yml b/book/_toc.yml
index 1c03d88..671dca2 100644
--- a/book/_toc.yml
+++ b/book/_toc.yml
@@ -59,6 +59,7 @@ parts:
         sections:
         - file: features/widgets.ipynb
         - file: features/exercise_checking.ipynb
+      - file: external/Sphinx-Image-Inverter/README.md
 
   - caption: Examples
     chapters:
diff --git a/book/external/Sphinx-Image-Inverter b/book/external/Sphinx-Image-Inverter
new file mode 160000
index 0000000..90af0ab
--- /dev/null
+++ b/book/external/Sphinx-Image-Inverter
@@ -0,0 +1 @@
+Subproject commit 90af0ab633f42e571d3e13aaba1f18160b4a83ac