[viewController.navigationItem.rightBarButtonItem setBackgroundVerticalPositionAdjustment:50 forBarMetrics:UIBarMetricsDefault];

UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];
[btn setBackgroundImage:[UIImage imageNamed:@"chk_back.png"] forState:UIControlStateNormal];
[btn addTarget:self action:@selector(BackBtn) forControlEvents:UIControlEventTouchUpInside];
UIView *customView = [[[UIView alloc] initWithFrame:CGRectMake(0,0, 50, 38)] autorelease];
btn.frame = CGRectMake(10, 3, 30, 32); // where you can set your insets
[customView addSubview:btn];
    UIBarButtonItem *barBtn = [[UIBarButtonItem alloc] initWithCustomView:customView];

UIBarButtonItem *fixedSpace = [[UIBarButtonItem alloc]
                   initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace
                                        target:nil
                                        action:nil];
[fixedSpace setWidth:20];