diff --git a/imagekit/__init__.py b/imagekit/__init__.py
index 2c72bf4..e65c270 100644
--- a/imagekit/__init__.py
+++ b/imagekit/__init__.py
@@ -1,6 +1,7 @@
 # flake8: noqa
 
 from . import conf
+from . import generatorlibrary
 from .specs import ImageSpec
 from .pkgmeta import *
 from .registry import register, unregister
diff --git a/imagekit/generatorlibrary.py b/imagekit/generatorlibrary.py
new file mode 100644
index 0000000..1786022
--- /dev/null
+++ b/imagekit/generatorlibrary.py
@@ -0,0 +1,13 @@
+from .registry import register
+from .processors import Thumbnail as ThumbnailProcessor
+from .specs import ImageSpec
+
+
+class Thumbnail(ImageSpec):
+    def __init__(self, width=None, height=None, anchor='auto', crop=True, **kwargs):
+        self.processors = [ThumbnailProcessor(width, height, anchor=anchor,
+                                              crop=crop)]
+        super(Thumbnail, self).__init__(**kwargs)
+
+
+register.spec('ik:thumbnail', Thumbnail)
diff --git a/imagekit/processors/resize.py b/imagekit/processors/resize.py
index 624ccf6..f241ca6 100644
--- a/imagekit/processors/resize.py
+++ b/imagekit/processors/resize.py
@@ -235,6 +235,9 @@ class Thumbnail(object):
 
     def process(self, img):
         if self.crop:
+            if not self.width or not self.height:
+                raise Exception('You must provide both a width and height when'
+                    ' cropping.')
             if self.anchor == 'auto':
                 processor = SmartResize(self.width, self.height)
             else: