diff --git a/README.rst b/README.rst
index f4e6d3c31cc50228c8b81c0b09831fef81b1c73c..fb803e03f98b36837d7a739513a37cd432f98476 100644
--- a/README.rst
+++ b/README.rst
@@ -57,6 +57,9 @@ Some extra dependencies are listed in `requirements.txt
   class has some methods which use ``trimesh`` to perform geometric queries
   on the mesh.
 
+- ``Pillow``: The `fsl.data.bitmap.Bitmap <fsl/data/bitmap.py`_ class uses
+  ``Pillow`` to load image files.
+
 
 If you are using Linux, you need to install wxPython first, as binaries are
 not available on PyPI. Install wxPython like so, changing the URL for your
diff --git a/requirements-extra.txt b/requirements-extra.txt
index 774efb03f31ee244da45acd703238d60c02191b8..333cde7a982fd7502eec2af60e26b87b332dac0f 100644
--- a/requirements-extra.txt
+++ b/requirements-extra.txt
@@ -2,3 +2,4 @@ indexed_gzip>=0.7.0
 wxpython==4.*
 trimesh>=2.37.29
 rtree==0.8.3
+Pillow>=3.2.0