diff --git a/wagtail/wagtailimages/utils.py b/wagtail/wagtailimages/utils.py
index 627a8f1215..ff80833107 100644
--- a/wagtail/wagtailimages/utils.py
+++ b/wagtail/wagtailimages/utils.py
@@ -4,6 +4,7 @@ from PIL import Image
 
 from django.core.exceptions import ValidationError
 from django.utils.translation import ugettext_lazy  as _
+from django.conf import settings
 
 
 def validate_image_format(f):
@@ -16,12 +17,26 @@ def validate_image_format(f):
     if extension not in ['gif', 'jpeg', 'png']:
         raise ValidationError(_("Not a valid image. Please use a gif, jpeg or png file with the correct file extension."))
 
+    if f.closed:
+        # Reopen the file
+        file = open(os.path.join(settings.MEDIA_ROOT, f.name), 'rb')
+        close = True
+    else:
+        # Seek to first byte but save position to be restored later
+        file_position = f.tell()
+        f.seek(0)
+        file = f
+        close = False
+
     # Open image file
-    file_position = f.tell()
-    f.seek(0)
-    image = Image.open(f)
-    f.seek(file_position)
+    image = Image.open(file)
 
     # Check that the internal format matches the extension
     if image.format.upper() != extension.upper():
         raise ValidationError(_("Not a valid %s image. Please use a gif, jpeg or png file with the correct file extension.") % (extension.upper()))
+
+    # Close/restore file
+    if close:
+        file.close()
+    else:
+        f.seek(file_position)