I got this Code from Base code itself, anyway this will be helpful for those who searching for default image in odoo 9.

In Header, 

from openerp.tools import openerp,image_colorize, image_resize_image_big

In Py,

photo = fields.Binary(string="Image to upload", default=lambda self:self._get_default_image())

@api.model

def _get_default_image(self, colorize=False):

image = image_colorize(open(openerp.modules.get_module_resource('base','static/src/img', 'avatar.png')).read())

return image_resize_image_big(image.encode('base64'))

In Xml, should specify widget="image" for showing image in Form.

<field name="photo" widget="image" />