I am trying to create a model for product images in Odoo 10. A product image can have one original uploaded image and two computed versions: medium and small. This is the code I wrote:

# -*- coding: utf-8 -*-
from odoo import models, fields, api, tools

class ProductImage(models.Model):
    _name = 'product_images.product_image'
    _description = 'Product image'

    name = fields.Char(string="Alternative text")
    product_id = fields.Many2one('product.product', string='Product', ondelete='set null', index=True)
    original_image = fields.Binary(string='Original image')
    medium_image = fields.Binary(string='Medium image', compute='_resize_image', store=True)
    small_image = fields.Binary(string='Small image', compute='_resize_image', store=True)

    @api.depends('original_image')
    def _resize_image(self):
        for record in self.with_context(bin_size=False):
            data = tools.image_get_resized_images(record.original_image)
            record.medium_image = data['image_medium']
            record.small_image = data['image_small']

When I change the original_image in the form view or in the Odoo shell, _resize_image() is called, but medium_image and small_image values are not saved. What do I miss?