- (void)viewDidLoad{
    __weak typeof(self) weakSelf = self;
    NSURL *url = [NSURL URLWithString:@"http://img4.duitang.com/uploads/item/201211/24/20121124112047_KUFxK.gif"];
    [[SDWebImageManager sharedManager]diskImageExistsForURL:url completion:^(BOOL isInCache) {
        __strong typeof(weakSelf) strongSelf = weakSelf;
        if (isInCache) {
            NSString *cacheImageKey = [[SDWebImageManager sharedManager]cacheKeyForURL:url];
            if (cacheImageKey.length) {
                NSString *cacheImagePath = [[SDImageCache sharedImageCache]defaultCachePathForKey:cacheImageKey];
                if (cacheImagePath.length) {
                    NSData *imageData = [NSData dataWithContentsOfFile:cacheImagePath];
                    [strongSelf writeGifImgToAlbum:imageData];
                }
            }
        }
    }];
}

- (void)writeGifImgToAlbum:(NSData *)gifData{
    //#import <AssetsLibrary/AssetsLibrary.h>
    ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
    //#import <MobileCoreServices/UTCoreTypes.h>
    NSDictionary *metadata = @{@"UTI":(__bridge NSString *)kUTTypeGIF};
    [library writeImageDataToSavedPhotosAlbum:gifData metadata:metadata completionBlock:^(NSURL *assetURL, NSError *error) {
        if (!error) {
            NSLog(@"保存成功");
        }else{
            NSLog(@"false");
        }
    }];
}

原文：http://www.jianshu.com/p/c63343516f32