bg_header.jpg was missing

This image is used in case there is no image defined for header_background_image on web site layout. So in this case, the image was not found and error was raised.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@31544 20353a03-c40f-0410-a6d1-a30d3c3de9de