Only show POD image if so configured; use "image not found" fallback

also update a random docstring

rattail/config.py

@@ -580,6 +580,10 @@ class RattailConfig(object):
         """
         Returns the title string to be used when displaying product key field,
         e.g. "UPC" or "Part No." etc.
+
+        :param key: The product key for which to return a label.  This
+           is optional; if not specified then :meth:`product_key()`
+           will be called to determine the key.
         """
         title = self.get('rattail', 'product.key_title')
         if title:

rattail/products.py

@@ -184,18 +184,30 @@ class ProductsHandler(GenericHandler):
         """
         Return the preferred image URL for the given UPC or product.
         """
-        # we prefer the "image on file" if available
+        base_url = self.config.base_url()
-        if product and product.image:
-            url = self.config.base_url()
-            if url:
-                return '{}/products/{}/image'.format(url, product.uuid)
 
-        # fallback to the POD image, if available
+        # we prefer the "image on file" if available
+        if base_url and product and product.image:
+            return '{}/products/{}/image'.format(base_url, product.uuid)
+
+        # and if this product is a pack item, then we prefer the unit
+        # item image as fallback, if available
+        if base_url and product and product.is_pack_item():
+            unit = product.unit
+            if unit and unit.image:
+                return '{}/products/{}/image'.format(base_url, unit.uuid)
+
+        # fallback to the POD image, if available and so configured
+        if self.config.getbool('tailbone', 'products.show_pod_image',
+                               default=False):
             if product and not upc:
                 upc = product.upc
             if upc:
                 return self.get_pod_image_url(upc)
 
+        if base_url:
+            return '{}/tailbone/img/product.png'.format(base_url)
+
     def get_pod_image_url(self, upc, **kwargs):
         """
         Return the POD image URL for the given UPC.