Fixed django#18374 -- Explained "corrupt image" error

Thanks fabian and charettes.

Author: aaugustin

docs/ref/forms/fields.txt

@@ -591,7 +591,11 @@ For each field, we describe the default widget used if you don't specify
  * Error message keys: ``required``, ``invalid``, ``missing``, ``empty``,
  ``invalid_image``
 
- Using an ImageField requires that the `Python Imaging Library`_ is installed.
+ Using an ``ImageField`` requires that the `Python Imaging Library`_ (PIL)
+ is installed and supports the image formats you use. If you encounter a
+ ``corrupt image`` error when you upload an image, it usually means PIL
+ doesn't understand its format. To fix this, install the appropriate
+ library and reinstall PIL.
 
  When you use an ``ImageField`` on a form, you must also remember to
  :ref:`bind the file data to the form <binding-uploaded-files>`.