PHImageManager - objcmemo @ ウィキ

typeof(self) __weak wself = self;
PHImageRequestOptions *options = [[PHImageRequestOptions alloc] init];
options.resizeMode = PHImageRequestOptionsResizeModeExact;
options.deliveryMode = PHImageRequestOptionsDeliveryModeHighQualityFormat;
 
[[PHImageManager defaultManager] requestImageForAsset:asset
                                           targetSize:CGSizeMake(1024, 1024)
                                          contentMode:PHImageContentModeAspectFit
                                              options:options
                                        resultHandler:^(UIImage *image, NSDictionary *info) {
                                            if (image) {
                                                NSLog(@"%@", NSStringFromCGSize(image.size));
                                                wself.imageView.image = image;
                                            }
                                        }];