Oprah Winfrey will help Weight Watchers International Inc pick a new leader, the diet company said on Monday, about a year after the media mogul announced she had bought a 10 percent stake in the company. Weight Watchers said its chief executive, James Chambers, would resign at the end of September after three years on the job. The company has struggled to compete with other weight-loss programs in recent years, and revenues in the quarter ended July 2 were little changed from a year earlier.