Try this my friend:
setting the buttons
adjustImageWhenHighlighted = NO;
should do the trick.
Hope it helps. Cheers
UPDATE:
// Initialize the UIButton
UIButton *customButton= [UIButton buttonWithType:UIButtonTypeCustom];
//normal_button.png and selected_button.png need to be created by you
[customButton setBackgroundImage: [UIImage imageNamed:@"normal_button.png"] forState:UIControlStateNormal];
[customButton setBackgroundImage: [UIImage imageNamed:@"normal_button.png"] forState:UIControlStateSelected];
UIBarButtonItem *barButtonItem = [[UIBarButtonItem alloc] initWithCustomView: customButton];
Another way to do this.