Use 'default_get' method to assign a default image.
Check these links for reference.

(docs)

https://www.odoo.com/documentation/8.0/reference/orm.html#openerp.models.Model.default_get

(example)

https://github.com/odoo/odoo/blob/8.0/addons/account/account.py#L3085

Hi,
For Odoo 10.
from odoo.modules.module import get_module_resource

product_image = fields.Binary(

        string='Global Product Image',

        default=lambda s: s._default_product_image(),

        help='Use as the global image for all product default images. '

             'Limited to 1024x1024.',

    )

    @api.model

    def _default_product_image(self):

        image_path = get_module_resource(

            'module_name', 'static/src/img', 'glob_prod_img.png'

        )

        with open(image_path, 'rb') as handler:

            image_data = handler.read()

        return tools.image_resize_image_big(

            image_data.encode('base64')

        )