Updating Your Image Resource Files

Apps running in iOS 4 should now include two separate files for each image resource. One file provides a standard-resolution version of a given image, and the second provides a high-resolution version of the same image. The naming conventions for each pair of image files is as follows:

Standard: <ImageName> <device_modifier>.<filename_extension>
High resolution: <ImageName>@2x<device_modifier>.<filename_extension>

The <ImageName> and <filename_extension> portions of each name specify the usual name and extension for the file. The <device_modifier> portion is optional and contains either the string ~ipad
or ~iphone. You include one of these modifiers when you want to specify different versions of an image for iPad and iPhone. The inclusion of the @2x
modifier for the high-resolution image is new and lets the system know that the image is the high-resolution variant of the standard image.
Important: The order of the modifiers is critical. If you incorrectly put the @2x after the device modifier, iOS will not find the image.

When creating high-resolution versions of your images, place the new versions in the same location in your app bundle as the original.

上面是引自苹果官方文档的一段话，详细介绍了如何命名图片文件。
eg:
image1@2x~ipad.png ,image1@3x~iphone.png
在代码中，如果要使用这张图片，应该
UIImage *image = [UIImage imageNamed:@"image1.png"]; 或

NSString *resourcePath = [[NSBundle mainBundle] resourcePath];
NSString *path = [resourcePath stringByAppendingPathComponent:@"image1.png"];
UIImage *image = [UIImage imageWithContentsOfFile:path];

此时系统会自动根据设备的分辨率及类型，自动选择正确的图片。
错误的写法：
[UIImage imageNamed:@"image1@2x.png"],[UIImage imageNamed:@"image1@2x~ipad.png"]

NSString *resourcePath = [[NSBundle mainBundle] resourcePath];
NSString *path = [resourcePath stringByAppendingPathComponent:@"image1@2x~ipad.png"];
UIImage *image = [UIImage imageWithContentsOfFile:path];

错误的写法虽然也可以拿到图片，但是并不会根据iPhone和ipad自动切换，2倍图3倍图也不会自动切换。

如果使用如下的写法，则文件名必须写全。

NSString *path = [[NSBundle mainBundle] pathForResource:@"image1@2x~ipad" ofType:@"png"];
UIImage *image = [UIImage imageWithContentsOfFile:path];

这是因为- (nullable NSString *)pathForResource:(nullable NSString *)name ofType:(nullable NSString *)ext;方法只是查找对应的资源文件，并不会自动补全，所以如果不写全文件名，则查找不到对应的文件，会返回nil。也即path = nil，所以如果不写全是不能得到对应的image。