Hi,

In Odoo 15 and 16 you can do like this:

from odoo import models


class ResPartner(models.Model):
    _inherit = 'res.partner'

    def _avatar_get_placeholder_path(self):
        # odoo_forum is the technical name of the custom_module
        if self.type == 'contact':
            return "odoo_forum/static/img/contact_placeholder.png"
        if self.type == 'private':
            return "odoo_forum/static/img/private_placeholder.png"
        if self.type == 'other':
            return "odoo_forum/static/img/other_placeholder.png"
        return super()._avatar_get_placeholder_path()

Regards