diff --git a/wagtail/wagtailimages/utils.py b/wagtail/wagtailimages/utils.py
index a521477ed..e9cbb4f56 100644
--- a/wagtail/wagtailimages/utils.py
+++ b/wagtail/wagtailimages/utils.py
@@ -5,8 +5,6 @@ import base64
 import hmac
 import hashlib
 
-from six import b
-
 from PIL import Image
 
 from django.core.exceptions import ValidationError
@@ -81,7 +79,7 @@ def generate_signature(image_id, filter_spec):
     # Based on libthumbor hmac generation
     # https://github.com/thumbor/libthumbor/blob/b19dc58cf84787e08c8e397ab322e86268bb4345/libthumbor/crypto.py#L50
     url = str(image_id) + '/' + str(filter_spec) + '/'
-    return base64.urlsafe_b64encode(hmac.new(b(settings.SECRET_KEY), b(url), hashlib.sha1).digest())
+    return base64.urlsafe_b64encode(hmac.new(settings.SECRET_KEY.encode(), url.encode(), hashlib.sha1).digest())
 
 
 def verify_signature(signature, image_id, filter_spec):
diff --git a/wagtail/wagtailimages/views/frontend.py b/wagtail/wagtailimages/views/frontend.py
index 80ab989ff..b5ccf6e6e 100644
--- a/wagtail/wagtailimages/views/frontend.py
+++ b/wagtail/wagtailimages/views/frontend.py
@@ -10,7 +10,7 @@ from wagtail.wagtailimages import image_processor
 def serve(request, signature, image_id, filter_spec):
     image = get_object_or_404(get_image_model(), id=image_id)
 
-    if not verify_signature(signature, image_id, filter_spec):
+    if not verify_signature(signature.encode(), image_id, filter_spec):
         raise PermissionDenied
 
     try: