You need to specify a stretchable UIImage
with UIEdgeInsets
as below:
UIImage *normalStateImage = [[UIImage imageNamed:@"image"] resizableImageWithCapInsets:UIEdgeInsetsMake(10.0, 10.0, 10.0, 10.0)];
UIImage *pressedStateImage = [[UIImage imageNamed:@"image-pressed"] resizableImageWithCapInsets:UIEdgeInsetsMake(10.0, 10.0, 10.0, 10.0)];
[[UIBarButtonItem appearance] setBackButtonBackgroundImage:normalStateImage forState:UIControlStateNormal barMetrics:UIBarMetricsDefault];
[[UIBarButtonItem appearance] setBackButtonBackgroundImage:pressedStateImage forState:UIControlStateHighlighted barMetrics:UIBarMetricsDefault];
iOS 6.x is clever enough to determine the image sizes and stretch according to it.