读取和保存图片
以下代码可以将图片存入本地的 Documents 下
//得到路径数组
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
//取得documents目录
NSString *documentsDirectory = [paths objectAtIndex:0];
//创建保留图片的地址
NSString *savedImagePath = [documentsDirectory stringByAppendingPathComponent:@"savedImage.png"];

UIImage *image = [UIImage imageNamed:@"resources.png"];
NSData *imageData = UIImagePNGRepresentation(image);

[imageData writeToFile:savedImagePath atomically:NO];