note:

I just create test app and test.

So for devices with out retina:

ImageName.png - For iPhone/iPod

ImageName~ipad.png -- For iPad

For devices with retina display:

ImageName@2x~ipad.png -- For iPad

ImageName@2x.png - For iPhone/iPod

And you can still use @2x if you iPhone high resolution image and iPad high resolution image have the same size.

To load image just use [UIImage imageNamed:@"image.png"];