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';