add '-flatten' when converting to opaque format like jpeg.

otherwise imagemagick will create black-background image when resizing.
http://studio.imagemagick.org/discourse-server/viewtopic.php?f=3&t=19677

product/ERP5/Document/Document.py

@@ -70,6 +70,7 @@ VALID_TEXT_FORMAT_LIST = ('text', 'txt', 'html', 'base_html',
                           'stripped-html')
 
 VALID_IMAGE_FORMAT_LIST = ('jpg', 'jpeg', 'png', 'gif', 'pnm', 'ppm', 'tiff')
+VALID_TRANSPARENT_IMAGE_FORMAT_LIST = ('png', 'gif', 'tiff')
 
 DEFAULT_DISPLAY_ID_LIST = ('nano', 'micro', 'thumbnail',
                             'xsmall', 'small', 'medium',

product/ERP5/Document/Image.py

@@ -44,7 +44,8 @@ from Products.ERP5Type import Permissions, PropertySheet
 from Products.ERP5Type.Utils import fill_args_from_request
 from Products.ERP5.Document.File import File
 from Products.ERP5.Document.Document import Document, ConversionError,\
-                     VALID_TEXT_FORMAT_LIST, DEFAULT_DISPLAY_ID_LIST, DEFAULT_IMAGE_QUALITY, _MARKER
+                     VALID_TEXT_FORMAT_LIST, VALID_TRANSPARENT_IMAGE_FORMAT_LIST,\
+                     DEFAULT_DISPLAY_ID_LIST, DEFAULT_IMAGE_QUALITY, _MARKER
 from os.path import splitext
 from OFS.Image import Image as OFSImage
 from OFS.Image import getImageInfo
@@ -310,6 +311,8 @@ class Image(TextConvertableMixin, File, OFSImage):
 
     parameter_list = ['convert']
     parameter_list.extend(['-colorspace', 'RGB'])
+    if format not in VALID_TRANSPARENT_IMAGE_FORMAT_LIST:
+      parameter_list.append('-flatten')
     if resolution:
       parameter_list.extend(['-density', '%sx%s' % (resolution, resolution)])
     parameter_list.extend(['-quality', str(quality)])