diff --git a/src/google/appengine/api/images/__init__.py b/src/google/appengine/api/images/__init__.py
index 1b40c3e..016af78 100755
--- a/src/google/appengine/api/images/__init__.py
+++ b/src/google/appengine/api/images/__init__.py
@@ -54,7 +54,12 @@
 from six.moves import range
 
 
-_USE_GRPC = bool(os.environ.get('USE_CUSTOM_IMAGES_GRPC_SERVICE'))
+# Image server configuration environment variable names.
+USE_CUSTOM_IMAGES_GRPC_SERVICE = 'APPENGINE_USE_CUSTOM_IMAGES_GRPC_SERVICE'
+IMAGES_SERVICE_ENDPOINT = 'APPENGINE_IMAGES_SERVICE_ENDPOINT'
+
+
+_USE_GRPC = bool(os.environ.get(USE_CUSTOM_IMAGES_GRPC_SERVICE))
 
 
 class _FakeRpc(object):
@@ -69,9 +74,10 @@ def get_result(self):
 
 def _make_grpc_call(method_name, request):
     """Creates an authenticated gRPC channel and makes an RPC call."""
-    endpoint = os.environ.get('IMAGES_SERVICE_ENDPOINT')
+    endpoint = os.environ.get(IMAGES_SERVICE_ENDPOINT)
     if not endpoint:
-        raise ValueError("IMAGES_SERVICE_ENDPOINT environment variable not set")
+        raise ValueError(
+            "%s environment variable not set" % IMAGES_SERVICE_ENDPOINT)
 
     parsed = six.moves.urllib.parse.urlparse(endpoint)
     target_host = parsed.netloc + ':443'
diff --git a/tests/google/appengine/api/images/images_test.py b/tests/google/appengine/api/images/images_test.py
index 2c776b3..0cfbfcc 100644
--- a/tests/google/appengine/api/images/images_test.py
+++ b/tests/google/appengine/api/images/images_test.py
@@ -27,16 +27,16 @@ class ImagesGrpcTest(absltest.TestCase):
 
   def setUp(self):
     super().setUp()
-    os.environ['USE_CUSTOM_IMAGES_GRPC_SERVICE'] = 'True'
+    os.environ[images.USE_CUSTOM_IMAGES_GRPC_SERVICE] = 'True'
     self.use_grpc_patcher = mock.patch('google.appengine.api.images._USE_GRPC', True)
     self.use_grpc_patcher.start()
-    os.environ['IMAGES_SERVICE_ENDPOINT'] = 'https://localhost'
+    os.environ[images.IMAGES_SERVICE_ENDPOINT] = 'https://localhost'
 
   def tearDown(self):
     self.use_grpc_patcher.stop()
     super().tearDown()
-    del os.environ['USE_CUSTOM_IMAGES_GRPC_SERVICE']
-    del os.environ['IMAGES_SERVICE_ENDPOINT']
+    del os.environ[images.USE_CUSTOM_IMAGES_GRPC_SERVICE]
+    del os.environ[images.IMAGES_SERVICE_ENDPOINT]
 
   @mock.patch('google.appengine.api.images._make_grpc_call')
   def test_execute_transforms_async_grpc(self, mock_make_grpc_call):