ios - Navigation bar with retina image -

ios - Navigation bar with retina image -

i'm trying add together image navigation bar, download image through internet. if take 320x44 image it's ok, if take 640x88 image, retina resolution, image in navigation bar not resized , appears out of bounds. set image code:

[[uinavigationbar appearance] setbackgroundimage:im forbarmetrics:uibarmetricsdefault];

ios xcode