diff --git a/sqlalchemy_imageattach/entity.py b/sqlalchemy_imageattach/entity.py
index 5c62e0f..31025b4 100644
--- a/sqlalchemy_imageattach/entity.py
+++ b/sqlalchemy_imageattach/entity.py
@@ -76,6 +76,7 @@ class User(Base):
 import io
 import numbers
 import shutil
+import threading
 import uuid
 
 from sqlalchemy import Column
@@ -96,7 +97,7 @@ class User(Base):
 from .context import current_store, get_current_store, store_context
 from .file import ReusableFileProxy
 from .store import Store
-from .util import append_docstring_attributes
+from .util import append_docstring_attributes, classproperty
 
 __all__ = ('VECTOR_TYPES', 'BaseImageSet', 'BaseImageQuery', 'Image',
            'ImageSet', 'ImageSubset', 'MultipleImageSet', 'SingleImageSet',
@@ -373,18 +374,32 @@ class BaseImageQuery(Query):
 
     """
 
-    #: (:class:`collections.abc.MutableSet`) The set of instances that their
-    #: image files are stored but the ongoing transaction isn't committed.
-    #: When the transaction might fail and rollback, image files in the
-    #: set are deleted back in the storage.
-    _stored_images = set()
-
-    #: (:class:`collections.abc.MutableSet`) The set of instanced marked
-    #: as deleted.  If the ongoing transaction is successfully committed
-    #: the actual files in the storages will be deleted as well.
-    #: When the transaction might fail and rollback, image files won't
-    #: deleted and the set will be empty.
-    _deleted_images = set()
+    _local = threading.local()
+
+    @classproperty
+    def _stored_images(cls):
+        """(:class:`collections.abc.MutableSet`) The set of instances that
+        their image files are stored but the ongoing transaction isn't
+        committed. When the transaction might fail and rollback, image files
+        in the set are deleted back in the storage. This set is thread-local.
+
+        """
+        if not getattr(cls._local, '_stored_images', None):
+            cls._local._stored_images = set()
+        return cls._local._stored_images
+
+    @classproperty
+    def _deleted_images(cls):
+        """(:class:`collections.abc.MutableSet`) The set of instanced marked
+        as deleted.  If the ongoing transaction is successfully committed
+        the actual files in the storages will be deleted as well. When the
+        transaction might fail and rollback, image files won't deleted and
+        the set will be empty. This set is thread-local.
+
+        """
+        if not getattr(cls._local, '_deleted_images', None):
+            cls._local._deleted_images = set()
+        return cls._local._deleted_images
 
     @classmethod
     def _mark_image_file_stored(cls, mapper, connection, target):
diff --git a/sqlalchemy_imageattach/util.py b/sqlalchemy_imageattach/util.py
index eb0a070..a2f2315 100644
--- a/sqlalchemy_imageattach/util.py
+++ b/sqlalchemy_imageattach/util.py
@@ -10,7 +10,7 @@
 import textwrap
 
 __all__ = ('append_docstring', 'append_docstring_attributes',
-           'get_minimum_indent')
+           'classproperty', 'get_minimum_indent')
 
 
 def get_minimum_indent(docstring, ignore_before=1):
@@ -102,3 +102,11 @@ class Entity(object):
             *lines
         )
     return docstring
+
+
+class classproperty(object):
+    def __init__(self, getter):
+        self.getter = getter
+
+    def __get__(self, instance, owner):
+        return self.getter(owner)
diff --git a/tests/util_test.py b/tests/util_test.py
index 59ab906..6f7811a 100644
--- a/tests/util_test.py
+++ b/tests/util_test.py
@@ -1,6 +1,7 @@
 from __future__ import absolute_import
 
-from sqlalchemy_imageattach.util import append_docstring, get_minimum_indent
+from sqlalchemy_imageattach.util import (append_docstring, classproperty,
+                                         get_minimum_indent)
 
 
 def test_minimum_indent():
@@ -52,3 +53,14 @@ def test_func():
 
            Appended docstring!
     '''.rstrip()
+
+
+def test_classproperty():
+
+    class Foo(object):
+        @classproperty
+        def bar(cls):
+            return 'baz'
+
+    assert Foo.bar == 'baz'
+    assert Foo().bar == 'baz'