This has been bugging me for a little while - and this is the best solution I could find...

- (void)viewDidLoad
{
[super viewDidLoad];
UIImage *backImage = [UIImage imageNamed:@"custom-image.png"];
UIButton *backButton = [UIButton buttonWithType:UIButtonTypeCustom];
backButton.frame = CGRectMake(0, 0, backImage.size.width, backImage.size.height);
[backButton setImage:backImage forState:UIControlStateNormal];
[backButton addTarget:self action:@selector(backButtonHandler:) forControlEvents:UIControlEventTouchUpInside];
UIBarButtonItem *backBarButtonItem = [[UIBarButtonItem alloc] initWithCustomView:backButton] ;
self.navigationItem.hidesBackButton = YES;
self.navigationItem.leftBarButtonItem = backBarButtonItem;
}
- (void)backButtonHandler:(id)sender
{
[self.navigationController popViewControllerAnimated:YES];
}