Defer image attachment

If the Word template does not use the images sent in the template context
information, there is no need to insert them into the generated document.

docxtpl/__init__.py

@@ -282,21 +282,32 @@ class Listing(object):
     def __str__(self):
         return self.xml
 
+
 class InlineImage(object):
-    """ class to generate an inline image
+    """Class to generate an inline image
 
     This is much faster than using Subdoc class.
     """
-    def __init__(self, tpl, file, width=None, height=None):
-        image_xml = tpl.docx._part.new_pic_inline(file, width, height).xml
-        self.xml = '</w:t></w:r><w:r><w:drawing>%s</w:drawing></w:r><w:r><w:t xml:space="preserve">' % image_xml
+    tpl = None
+    image_descriptor = None
+    width = None
+    height = None
+
+    def __init__(self, tpl, image_descriptor, width=None, height=None):
+        self.tpl, self.image_descriptor = tpl, image_descriptor
+        self.width, self.height = width, height
+
+    def _insert_image(self):
+        pic = self.tpl.docx._part.new_pic_inline(
+            self.image_descriptor,
+            self.width,
+            self.height
+        ).xml
+        return '</w:t></w:r><w:r><w:drawing>%s</w:drawing></w:r><w:r>' \
+               '<w:t xml:space="preserve">' % pic
 
     def __unicode__(self):
-        return self.xml
+        return self._insert_image()
 
     def __str__(self):
-        return self.xml
-
-
-
-
+        return self._insert_image()