[sql]DELETE FROM attachment WHERE dateline<xxxx[/sql]
just replace the datline with the unixtimestamp of a day 60 days ago
this would delete all attachments older than that date, but if you want it just in a specific forum, it would be harder, you'd have to write a script which gets the postids of those forums and so on and so on, just with phpmyadmin i think it isn't possible