diff --git a/stdimage/fields.py b/stdimage/fields.py
index e0ec390..ef5b567 100644
--- a/stdimage/fields.py
+++ b/stdimage/fields.py
@@ -4,7 +4,7 @@
 from django.conf import settings
 from django.core.files.storage import FileSystemStorage
 from django.db.models import signals
-from django.db.models.fields.files import ImageField
+from django.db.models.fields.files import ImageField, FileDescriptor
 
 from forms import StdImageFormField
 from widgets import DelAdminFileWidget
@@ -29,9 +29,15 @@ def url(self):
     def size(self):
         return self.storage.size(self.name)
 
-class StdImageFileDescriptor( ImageFileDescriptor ):
+class StdImageFileDescriptor( FileDescriptor ):
     def __set__( self, instance, value ):
+        previous_file = instance.__dict__.get( self.field.name )
         super( StdImageFileDescriptor, self ).__set__( instance, value )
+        if previous_file is not None:
+            self.field.update_dimension_fields( instance, force = True )
+            signals.post_init.connect( self.field.update_dimension_fields, sender = instance.__class__ )
+            signals.post_save.connect( self.field._rename_resize_image, sender = instance.__class__ )
+            signals.post_init.connect( self.field._set_thumbnail, sender = instance.__class__ )
         self.field._set_thumbnail( instance )
 
 class StdImageField(ImageField):
@@ -42,7 +48,7 @@ class StdImageField(ImageField):
 
     """
     descriptor_class = StdImageFileDescriptor
-    def __init__(self, size=None, thumbnail_size=None, *args, **kwargs):
+    def __init__(self, *args, **kwargs):
         """Added fields:
             - size: a tuple containing width and height to resize image, and
             an optional boolean setting if is wanted forcing that size (None for not resizing).
@@ -53,6 +59,9 @@ def __init__(self, size=None, thumbnail_size=None, *args, **kwargs):
             (None for not creating a thumbnail
 
         """
+        size = kwargs.pop('size', None)
+        thumbnail_size = kwargs.pop('thumbnail_size', None)
+
         params_size = ('width', 'height', 'force')
         for att_name, att in (('size', size),
                               ('thumbnail_size', thumbnail_size)):
@@ -100,6 +109,16 @@ def _resize_image(self, filename, size):
         img = Image.open(filename)
         if (img.size[WIDTH] > size['width'] or
             img.size[HEIGHT] > size['height']):
+
+            #If the image is big resize it with the cheapest resize algorithm
+            factor = 1
+            while (img.size[0]/factor > 2*size['width'] and
+                   img.size[1]*2/factor > 2*size['height']):
+                factor *=2
+            if factor > 1:
+                img.thumbnail((int(img.size[0]/factor),
+                               int(img.size[1]/factor)), Image.NEAREST)
+
             if size['force']:
                 img = ImageOps.fit(img, (size['width'], size['height']),
                                    Image.ANTIALIAS)