diff --git a/scrapy/contrib/pipeline/images.py b/scrapy/contrib/pipeline/images.py
index 406d37522..77e9afb98 100644
--- a/scrapy/contrib/pipeline/images.py
+++ b/scrapy/contrib/pipeline/images.py
@@ -136,6 +136,7 @@ class BaseImagesPipeline(MediaPipeline):
             self.store_image(key, image, buf, info)
             if first_buf is None:
                 first_buf = buf
+        first_buf.seek(0)
         return md5sum(first_buf)
 
     def get_images(self, response, request, info):
diff --git a/scrapy/contrib/pipeline/s3images.py b/scrapy/contrib/pipeline/s3images.py
index 7f06233bd..a1018ee4c 100644
--- a/scrapy/contrib/pipeline/s3images.py
+++ b/scrapy/contrib/pipeline/s3images.py
@@ -3,7 +3,7 @@ import rfc822
 from scrapy.http import Request
 from scrapy.core.engine import scrapyengine
 from scrapy.core.exceptions import NotConfigured
-from scrapy.contrib.pipeline.images import BaseImagesPipeline, md5sum
+from scrapy.contrib.pipeline.images import BaseImagesPipeline
 from scrapy.conf import settings
 
 
diff --git a/scrapy/utils/misc.py b/scrapy/utils/misc.py
index 5c3c4b89c..3fe1e4122 100644
--- a/scrapy/utils/misc.py
+++ b/scrapy/utils/misc.py
@@ -64,18 +64,17 @@ def extract_regex(regex, text, encoding):
     else:
         return [remove_entities(unicode(s, encoding), keep=['lt', 'amp']) for s in strings]
 
-def md5sum(buffer):
-    """Calculate the md5 checksum of a file
+def md5sum(file):
+    """Calculate the md5 checksum of a file-like object without reading its
+    whole content in memory.
 
     >>> from StringIO import StringIO
     >>> md5sum(StringIO('file content to hash'))
     '784406af91dd5a54fbb9c84c2236595a'
-
     """
     m = hashlib.md5()
-    buffer.seek(0)
     while 1:
-        d = buffer.read(8096)
+        d = file.read(8096)
         if not d:
             break
         m.update(d)