diff --git a/openerp/tools/image.py b/openerp/tools/image.py
index 4473641b0f6..b64647dbb11 100644
--- a/openerp/tools/image.py
+++ b/openerp/tools/image.py
@@ -111,16 +111,16 @@ def image_save_for_web(image, fp=None, format=None):
     opt = dict(format=image.format or format)
     if image.format == 'PNG':
         opt.update(optimize=True)
+        alpha = False
+        if image.mode in ('RGBA', 'LA') or (image.mode == 'P' and 'transparency' in image.info):
+            alpha = image.convert('RGBA').split()[-1]
         if image.mode != 'P':
             # Get the alpha band
             alpha = image.split()[-1]
             # Floyd Steinberg dithering by default
             image = image.convert('RGBA').convert('P', palette=Image.WEB, colors=256)
-            # Set all pixel values below 128 to 255 and the rest to 0
-            mask = Image.eval(alpha, lambda a: 255 if a <=128 else 0)
-            # Paste the color of index 255 and use alpha as a mask
-            image.paste(255, mask)
-            opt.update(transparency=255)
+        if alpha:
+            image.putalpha(alpha)
     elif image.format == 'JPEG':
         opt.update(optimize=True, quality=80)
     if fp: