The default solvings are not appropriate:
Here's an example to create a custom leftBarButtonItem:
UIImage *buttonImage = [UIImage imageNamed:@"backbutton.png"];
UIButton *aButton = [UIbutton buttonWithType:UIButtonTypeCustom];
[aButton setImage:buttonImage forState:UIControlStateNormal];
aButton.frame = CGRectMake(0.0,0.0,buttonImage.size.width,buttonImage.size.height);
[aButton addTarget:self action:@selector(aSelector) forControlEvents:UIControlEventTouchUpInside];
UIBarButtonItem *backButton = [[UIBarButtonItem alloc] initWithCustomView:aButton];
self.navigationItem.leftButtonItem = backButton;
Hope it helps...
Edit:
Try this...
UIBarButtonItem *backButton = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"yourimage.jpg"] style:UIBarButtonItemStyleBordered target:self action:@selector(aSelector)];
self.navigationItem.leftBarButtonItem = backButton;
I don't remember what type of button is the backbutton, try to change the default style to other. Good luck!