Zde je pracovní příklad pro vás:
CREATE TABLE test
(
id int auto_increment primary key,
details varchar(250)
);
INSERT INTO test (details) VALUES ('<img src="http://www.test.com/quiz_images/poster_from_postermywall.jpg" alt="" align="" border="0px">');
select substr(
substr(details, locate('src="', details) + 5),
1,
locate('"', substr(details, locate('src="', details) + 5)) - 1
) as 'src'
from test;
SQL Fiddle:spustitelné demo