Check this link:
http://jdbc.postgresql.org/documentation/84/binary-data.html
That I found because of this other link:
http://docs.jboss.org/hibernate/orm/3.5/api/org/hibernate/dialect/PostgreSQLDialect.html
By the way, can you check if you are using JPA 1 or 2?
As far as I know jbpm5.3 should work with postgreSQL
Cheers