我就是这样，对我来说效果很好:-)

I just did it like this, worked fine for me :-)

UIImage *backButtonImage = [UIImage imageNamed:@"navigationBarBack.png"];
UIButton *backButton = [UIButton buttonWithType:UIButtonTypeCustom];

[backButton setImage:backButtonImage
            forState:UIControlStateNormal];

backButton.frame = CGRectMake(0, 0, backButtonImage.size.width, backButtonImage.size.height);

[backButton addTarget:self
               action:@selector(popViewController)
     forControlEvents:UIControlEventTouchUpInside];

UIBarButtonItem *backBarButtonItem = [[UIBarButtonItem alloc] initWithCustomView:backButton];
self.navigationItem.leftBarButtonItem = backBarButtonItem;