Placez les opérations fastidieuses dans les threads non principaux et celles qui nécessitent des mises à jour de l'interface utilisateur dans le thread principal.

__weak typeof(self) weakSelf = self;
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // not in main block task
    [weakSelf getImageForCollectionView];
    dispatch_async(dispatch_get_main_queue(), ^{
        // main block. change ui
        NSLog(@"%@", weakSelf.photos);
    });
});

EDIT :

- (void)getImageForCollectionView:(void(^)(void))callback {
    _library = [[ALAssetsLibrary alloc] init];
    self.photos = [NSMutableDictionary dictionary];
    
    dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        [_library enumerateGroupsWithTypes:ALAssetsGroupAll usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
            if (group) {
                NSMutableArray *array = [NSMutableArray array];
                [group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) {
                    if (result) {
                        [array addObject:result];
                    }
                }];
                [self.photos setValue:array forKey:[group valueForProperty:@"ALAssetsGroupPropertyName"]];
            }
        } failureBlock:^(NSError *error) {
            
        }];
        dispatch_async(dispatch_get_main_queue(), ^{
            callback();
        });
    });
}

[self getImageForCollectionView:^{
    // ...
    NSLog(@"%@", self.photos);
}];