There's the "vacuumlo" contrib module that removes orphaned
large objects.
Thanks for pointing out. Vacuumlo looks at oid and lo columns to find orphaned lobs, but the idea could be easily adapted/extended to looks for image columns.
Why don't you use bytea instead of large objects in the database? That way you won't have to worry about orphaned large objects, and you don't have to convert to bytea upon retrieval.
When I started coding PostPic I knew little about PostgreSQL's large objects implementation, and was used to more 'traditional' BLOB types found in other databases.