Docs for format and jpegquality image filters

This also fixes #1205

docs/topics/images.rst

@@ -70,7 +70,7 @@ The available resizing methods are as follows:
 
             {% image page.photo height-480 %}
 
-        Resize the height of the image to the dimension specified.. 
+        Resize the height of the image to the dimension specified.
 
     ``fill``
         (takes two dimensions and an optional ``-c`` parameter)
@@ -197,3 +197,63 @@ Wagtail comes with three pre-defined image formats, but more can be defined in P
     The CSS classes added to images do **not** come with any accompanying stylesheets, or inline styles. e.g. the ``left`` class will do nothing, by default. The developer is expected to add these classes to their front end CSS files, to define exactly what they want ``left``, ``right`` or ``full-width`` to mean.
 
 For more information about image formats, including creating your own, see :ref:`rich_text_image_formats`
+
+Output image format
+-------------------
+
+Wagtail may automatically change the format of some images when they are resized:
+
+ - PNG and JPEG images always won't change format
+ - GIF images without animation are converted to PNGs
+ - BMP images are always converted to PNGs
+
+It is also possible to override the output format on a per-tag basis by using the
+``format`` filter after the resize rule.
+
+For example, to make the tag always convert the image to a JPEG, use ``format-jpeg``:
+
+.. code-block:: html+Django
+
+    {% image page.photo width-400 format-jpeg %}
+
+You may also use ``format-png`` or ``format-gif``.
+
+JPEG image quality
+------------------
+
+Wagtail's JPEG image quality setting defaults to 85 (which is quite high). This
+can be changed either globally or on a per-tag basis.
+
+Changing globally
+^^^^^^^^^^^^^^^^^
+
+Use the ``WAGTAILIMAGES_JPEG_QUALITY`` setting to change the global default JPEG
+quality:
+
+.. code-block:: python
+
+    # settings.py
+
+    # Make low-quality but small images
+    WAGTAILIMAGES_JPEG_QUALITY = 40
+
+Note that this won't affect any previously generated images. You may want to
+delete all renditions so they can regenerate with the new setting.
+
+Changing per-tag
+^^^^^^^^^^^^^^^^
+
+It's also possible to have different JPEG qualities on individual tags by using
+the ``jpegquality`` filter. This will always override the default setting:
+
+.. code-block:: html+Django
+
+    {% image page.photo width-400 jpegquality-40 %}
+
+Note that this will have no effect on PNG or GIF files. If you want all images
+to be low quality, you can use this filter with ``format-jpeg`` (which forces
+all images to output in JPEG format):
+
+.. code-block:: html+Django
+
+    {% image page.photo width-400 format-jpeg jpegquality-40 %}