There  are two built-in registered functions, lo_import
     and lo_export which  are  convenient  for  use
    in  SQL
     queries.
     Here is an example of their use
CREATE TABLE image (
    name            text,
    raster          oid
);
INSERT INTO image (name, raster)
    VALUES ('beautiful image', lo_import('/etc/motd'));
SELECT lo_export(image.raster, '/tmp/motd') FROM image
    WHERE name = 'beautiful image';