--- a/taskcluster/taskgraph/task/base.py
+++ b/taskcluster/taskgraph/task/base.py
@@ -1,15 +1,26 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 from __future__ import absolute_import, print_function, unicode_literals
 
 import abc
+import json
+import os
+import urllib2
+
+
+# if running in a task, prefer to use the taskcluster proxy (http://taskcluster/),
+# otherwise hit the services directly
+if os.environ.get('TASK_ID'):
+    INDEX_URL = 'http://taskcluster/index/v1/task/{}'
+else:
+    INDEX_URL = 'https://index.taskcluster.net/v1/task/{}'
 
 
 class Task(object):
     """
     Representation of a task in a TaskGraph.  Each Task has, at creation:
 
     - kind: the name of the task kind
     - label; the label for this task
@@ -23,33 +34,36 @@ class Task(object):
 
     A kind represents a collection of tasks that share common characteristics.
     For example, all build jobs.  Each instance of a kind is intialized with a
     path from which it draws its task configuration.  The instance is free to
     store as much local state as it needs.
     """
     __metaclass__ = abc.ABCMeta
 
-    def __init__(self, kind, label, attributes, task):
+    def __init__(self, kind, label, attributes, task, index_paths=None):
         self.kind = kind
         self.label = label
         self.attributes = attributes
         self.task = task
 
         self.task_id = None
         self.optimized = False
 
         self.attributes['kind'] = kind
 
+        self.index_paths = index_paths or ()
+
     def __eq__(self, other):
         return self.kind == other.kind and \
             self.label == other.label and \
             self.attributes == other.attributes and \
             self.task == other.task and \
-            self.task_id == other.task_id
+            self.task_id == other.task_id and \
+            self.index_paths == other.index_paths
 
     @classmethod
     @abc.abstractmethod
     def load_tasks(cls, kind, path, config, parameters, loaded_tasks):
         """
         Load the tasks for a given kind.
 
         The `kind` is the name of the kind; the configuration for that kind
@@ -85,18 +99,28 @@ class Task(object):
         it should be replaced with.
 
         The return value is a tuple `(optimized, taskId)`.  If `optimized` is
         true, then the task will be optimized (in other words, not included in
         the task graph).  If the second argument is a taskid, then any
         dependencies on this task will isntead depend on that taskId.  It is an
         error to return no taskId for a task on which other tasks depend.
 
-        The default never optimizes.
+        The default optimizes when a taskId can be found for one of the index
+        paths attached to the task.
         """
+        for index_path in self.index_paths:
+            try:
+                url = INDEX_URL.format(index_path)
+                existing_task = json.load(urllib2.urlopen(url))
+
+                return True, existing_task['taskId']
+            except urllib2.HTTPError:
+                pass
+
         return False, None
 
     @classmethod
     def from_json(cls, task_dict):
         """
         Given a data structure as produced by taskgraph.to_json, re-construct
         the original Task object.  This is used to "resume" the task-graph
         generation process, for example in Action tasks.

--- a/taskcluster/taskgraph/task/docker_image.py
+++ b/taskcluster/taskgraph/task/docker_image.py
@@ -1,16 +1,15 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 from __future__ import absolute_import, print_function, unicode_literals
 
 import logging
-import json
 import os
 import urllib2
 
 from . import base
 from taskgraph.util.docker import (
     docker_image,
     generate_context_hash,
     INDEX_PREFIX,
@@ -19,32 +18,22 @@ from taskgraph.util.templates import Tem
 
 logger = logging.getLogger(__name__)
 GECKO = os.path.realpath(os.path.join(__file__, '..', '..', '..', '..'))
 
 # if running in a task, prefer to use the taskcluster proxy (http://taskcluster/),
 # otherwise hit the services directly
 if os.environ.get('TASK_ID'):
     ARTIFACT_URL = 'http://taskcluster/queue/v1/task/{}/artifacts/{}'
-    INDEX_URL = 'http://taskcluster/index/v1/task/{}'
 else:
     ARTIFACT_URL = 'https://queue.taskcluster.net/v1/task/{}/artifacts/{}'
-    INDEX_URL = 'https://index.taskcluster.net/v1/task/{}'
 
 
 class DockerImageTask(base.Task):
 
-    def __init__(self, *args, **kwargs):
-        self.index_paths = kwargs.pop('index_paths')
-        super(DockerImageTask, self).__init__(*args, **kwargs)
-
-    def __eq__(self, other):
-        return super(DockerImageTask, self).__eq__(other) and \
-               self.index_paths == other.index_paths
-
     @classmethod
     def load_tasks(cls, kind, path, config, params, loaded_tasks):
         parameters = {
             'pushlog_id': params.get('pushlog_id', 0),
             'pushdate': params['moz_build_date'],
             'pushtime': params['moz_build_date'][8:],
             'year': params['moz_build_date'][0:4],
             'month': params['moz_build_date'][4:6],
@@ -94,32 +83,28 @@ class DockerImageTask(base.Task):
                              index_paths=index_paths))
 
         return tasks
 
     def get_dependencies(self, taskgraph):
         return []
 
     def optimize(self, params):
-        for index_path in self.index_paths:
+        optimized, taskId = super(DockerImageTask, self).optimize(params)
+        if optimized and taskId:
             try:
-                url = INDEX_URL.format(index_path)
-                existing_task = json.load(urllib2.urlopen(url))
                 # Only return the task ID if the artifact exists for the indexed
-                # task.  Otherwise, continue on looking at each of the branches.  Method
-                # continues trying other branches in case mozilla-central has an expired
-                # artifact, but 'project' might not. Only return no task ID if all
-                # branches have been tried
+                # task.
                 request = urllib2.Request(
-                    ARTIFACT_URL.format(existing_task['taskId'], 'public/image.tar.zst'))
+                    ARTIFACT_URL.format(taskId, 'public/image.tar.zst'))
                 request.get_method = lambda: 'HEAD'
                 urllib2.urlopen(request)
 
                 # HEAD success on the artifact is enough
-                return True, existing_task['taskId']
+                return True, taskId
             except urllib2.HTTPError:
                 pass
 
         return False, None
 
     @classmethod
     def from_json(cls, task_dict):
         # Generating index_paths for optimization