void RunTransaction(void Function(...))
{
    using (var manager = new NHibernateManager())
    {
        var session = manager.GetSession();
        using (var transaction = session.BeginTransaction())
        {
            Function();
            transaction.Commit();
        }
    }
}

public delegate void TransactionParticipant();

public void RunTransaction(Action fn)
{
   // ...
   fn();
   // ...
}

RunTransaction(() => Edit(editModel, uploadedImage));

public delegate void TransactionParticipant(NHibernateSession session);

fn(session);

RunTransaction(s => Edit(s, editModel, uploadedImage));

void RunTransaction(System.Action anAction)
{
  //...
  anAction();
  //...
}

void SomeMethod()
{
}

RunTransaction(SomeMethod);
RunTransaction(() => { /* any code */ });

void RunTransaction()//тут указывать параметром void_функцию незачем
{
  using (var manager = new NHibernateManager())
  {
    var session = manager.GetSession();
    using (var transaction = session.BeginTransaction())
    {
      Function();//!ф-ия будет вызываться просто если её "видно"
      transaction.Commit();
    }
  }
}