diff --git a/pixelocr/image.py b/pixelocr/image.py
index 3419556..d115486 100644
--- a/pixelocr/image.py
+++ b/pixelocr/image.py
@@ -45,15 +45,32 @@ class Image(object):
 
     @cached_property
     def bitmap(self):
+        """Return a two-color version of the image.
+
+        0 = white (blank) pixel
+        1 = black (letter) pixel
+        """
+
         grayscale = rgb2gray(self._data)
         return (grayscale < 1).astype('b')
 
     @property
     def key(self):
+        """Return a byte string uniquely representing the image."""
+        return self.tostring()
+
+    def tostring(self):
+        """Serialize the image as a string."""
         height, width, *_ = self.shape
-        shape = '{}x{}'.format(height, width)
+        shape = '{}x{}'.format(height, width).encode('latin1')
         bitmap = np.packbits(self.bitmap).tostring()
-        return shape.encode('latin1') + b':' + bitmap
+        return shape + b':' + bitmap
+
+    @classmethod
+    def fromstring(cls, data):
+        """Deserialize an image from a string."""
+        # TODO
+        raise NotImplementedError
 
     def unframe(self, width=2):
         return self[width:-width,width:-width]