I used (320 \* 44) pixels image size in navigation bar but seems blur in iPhone 5s or iPhone 4s?

Using code is:

```
[self.navigationController.navigationBar setBackgroundImage:[UIImage imageNamed:@"nav_Img.png"] forBarMetrics:UIBarMetricsDefault]; 

```

Anyone Please suggest me, what Size and resolution of Images which is easily fit in navigation bar for iPhone 5s or IPhone 4s?

Thanks in advance.