diff --git a/imagekit/templatetags/imagekit_tags.py b/imagekit/templatetags/imagekit_tags.py
index 92eefb5..0cb21ed 100644
--- a/imagekit/templatetags/imagekit_tags.py
+++ b/imagekit/templatetags/imagekit_tags.py
@@ -94,7 +94,12 @@ def spec(parser, token):
 spec = spec_tag = register.tag(spec)
 
 
-def _register_spec(id, spec):
+def _register_spec(id, spec=None):
+    if not spec:
+        def decorator(cls):
+            spec_registry.register(id, cls)
+            return cls
+        return decorator
     spec_registry.register(id, spec)