add image to UIBarButtonItem using initWithImage:(UIImage *)image

And for iOS 7+ you do the following:

Objective-C

 UIImage *image = [[UIImage imageNamed:@"myImage.png"] imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal];
 UIBarButtonItem *button = [[UIBarButtonItem alloc] initWithImage:image style:UIBarButtonItemStylePlain target:self action:@selector(YOUR_METHOD:)];

Swift 2.3

 let image = UIImage(named: "myImage")?.imageWithRenderingMode(.AlwaysOriginal)
 let button = UIBarButtonItem(image: image, style: .Plain, target: self, action: #selector(YOUR_METHOD(_:)))

Swift 3.0

let image = UIImage(named: "myImage")?.withRenderingMode(.alwaysOriginal)
let button = UIBarButtonItem(image: image, style: .plain, target: self, action: #selector(YOUR_METHOD(_:)))