Solution 1 (Direct assigning Base64)

Please follow the below three simple steps to set default image for a binary field.

Step 1 : Convert  image file to base64

        CONVERT ONLINE 

You can convert image file online. Example of Converted code ..

data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAIAAACQd1PeAAAA.....................

Step 2 : Remove the prifix 'data:image/png;base64,'

Example code after removing.

iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAIAAACQd1PeAAAA.....................

Step 3 : Assign Default Image

DEFAULT_IMG = 'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAIAAACQd1PeAAAA.....................'

class MyModel(models.Model):
    _name = 'my.model'

    field_binary = fields.Binary(default=DEFAULT_IMG)

This will set the default image :)

Solution 2 (Include image file into the module)

Step 1 : Copy file into module.

Eg ..      /my_module/static/img/my_image.png

Step 2 : Set Default

import base64
from odoo import models, fields
from odoo import modules


def get_default_img():
    with open(modules.get_module_resource('my_module', 'static/img', 'my_image.png'),
              'rb') as f:
        return base64.b64encode(f.read())

class MyModel(models.Model):
    _name = 'my.model'

    field_binary = fields.Binary(default=get_default_img())