diff --git a/boards/thumbs.py b/boards/thumbs.py
--- a/boards/thumbs.py
+++ b/boards/thumbs.py
@@ -51,13 +51,13 @@ def generate_thumb(img, thumb_size, form
         image2 = image
         image2.thumbnail(thumb_size, Image.ANTIALIAS)
 
-    string_io = io.StringIO()
+    output = io.BytesIO()
     # PNG and GIF are the same, JPG is JPEG
     if format.upper() == 'JPG':
         format = 'JPEG'
 
-    image2.save(string_io, format)
-    return ContentFile(string_io.getvalue())
+    image2.save(output, format)
+    return ContentFile(output.getvalue())
 
 
 class ImageWithThumbsFieldFile(ImageFieldFile):