Используйте этот код:
UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];
button.frame = CGRectMake(0,0,36,30);
[button setBackgroundImage:[UIImage imageNamed:@"backgroundImage.png"] forState:UIControlStateNormal];
[button addTarget:self.navigationController action:@selector(popViewControllerAnimated:) forControlEvents:UIControlEventTouchUpInside];
UIBarButtonItem *barButtonItem = [[UIBarButtonItem alloc] initWithCustomView:button];
[self.navigationItem setLeftBarButtonItem:barButtonItem];
Или используйте эту библиотеку. Очень легкий и красивый результат.
https://github.com/boctor/idev-recipes/tree/master/CustomBackButton