Hello 

try with below code, this will helps you.

from odoo import api, fields, models,  tools
from odoo.modules import get_module_resource
import base64

class your_class(models.Model):
    _name = 'your.class'
    
    @api.model
    def _get_default_image_value(self):
        image = False
        img_path = get_module_resource('your_modulename', 'static/img', 'avatar.png') # your default image path
        if img_path:
            with open(img_path, 'rb') as f: # read the image from the path
                image = f.read()
        if image: # if the file type is .jpg then you don't need this whole if condition.
            image = tools.image_colorize(image) 
        return tools.image_resize_image_big(base64.b64encode(image))
    
    @api.model
    def create(self, vals):
        if not vals.get('image'):
            vals['image'] = self._get_default_image_value()
        return super(your_class, self).create(vals)
    
    @api.multi
    def write(self, vals):
        res = super(your_class, self).write(vals)
        if not self.image:
            self.image = self._get_default_image_value()
        return res