Currently, to get the product image as url, we could use:

from odoo.tools.image import image_data_uri

product_image = image_data_uri(product.image_1920)

In my case,  product.image_1920 is the column of saved product image, you can replace it with your column to make it work.

Note: after get  product_image, you can see it still look like base64 encrypted format, but it's not, try to paste it to web browser such as Chrome, and you will see the image appear.

To resolve the 

TypeError: 'Binary' object is not subscriptable

error, use the decode('ascii') method:

with open("image.jpg", "rb") as img:
    b64_image = base64.b64encode(img.read())
    data = {'image_1920': b64_image.decode('ascii')}

Thanks Alain. The code I am using to write through the 

execute('res.partner', 'write', [6], {'image': imageBase64})

 Or using OdooRPC

model.browse(6).write({'image': imageBase64)

In both cases I get the following error:

xmlrpc.client.Fault: <Fault 'Binary' object is not subscriptable: 'Traceback (most recent call last):
  File "/home/odoo/odoo12/odoo/odoo/addons/base/controllers/rpc.py", line 60, in xmlrpc_1
    response = self._xmlrpc(service)
  File "/home/odoo/odoo12/odoo/odoo/addons/base/controllers/rpc.py", line 49, in _xmlrpc
    result = dispatch_rpc(service, method, params)
  File "/home/odoo/odoo12/odoo/odoo/http.py", line 120, in dispatch_rpc
    result = dispatch(method, params)
  File "/home/odoo/odoo12/odoo/odoo/service/model.py", line 40, in dispatch
    res = fn(db, uid, *params)
  File "/home/odoo/odoo12/odoo/odoo/service/model.py", line 98, in wrapper
    return f(dbname, *args, **kwargs)
  File "/home/odoo/odoo12/odoo/odoo/service/model.py", line 185, in execute
    res = execute_cr(cr, uid, obj, method, *args, **kw)
  File "/home/odoo/odoo12/odoo/odoo/service/model.py", line 169, in execute_cr
    result = odoo.api.call_kw(recs, method, args, kw)
  File "/home/odoo/odoo12/odoo/odoo/api.py", line 759, in call_kw
    return _call_kw_multi(method, model, args, kwargs)
  File "/home/odoo/odoo12/odoo/odoo/api.py", line 746, in _call_kw_multi
    result = method(recs, *args, **kwargs)
  File "/home/odoo/odoo12/odoo/addons/partner_autocomplete/models/res_partner.py", line 183, in write
    res = super(ResPartner, self).write(values)
  File "/home/odoo/odoo12/odoo/odoo/addons/base/models/res_partner.py", line 563, in write
    tools.image_resize_images(vals, sizes={'image': (1024, None)})
  File "/home/odoo/odoo12/odoo/odoo/tools/image.py", line 302, in image_resize_images
    vals.update(image_get_resized_images(vals[big_name],
  File "/home/odoo/odoo12/odoo/odoo/tools/image.py", line 292, in image_get_resized_images
    return_dict[big_name] = image_resize_image_big(base64_source, avoid_if_small=avoid_resize_big, size=size_big)
  File "/home/odoo/odoo12/odoo/odoo/tools/image.py", line 168, in image_resize_image_big
    return image_resize_image(base64_source, size, encoding, filetype, avoid_if_small)
  File "/home/odoo/odoo12/odoo/odoo/tools/image.py", line 66, in image_resize_image
    if size == (None, None) or base64_source[:1] == b'P':
TypeError: 'Binary' object is not subscriptable

Any advice to overcome this problem please?
Thanks Andres