gistfile1.m

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    static NSString *CellIdentifier = @"Cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

    NSString *imageURL = @"http://..."; //image URL gotten from Activity object

    //get a reference to the image view
    UIImageView *imageView = (UIImageView *)[cell viewWithTag:1]; //for example

    //using the SDWebImage downloader, start downloading the image
    SDWebImageManager *manager = [SDWebImageManager sharedManager];
        [manager downloadWithURL:[NSURL URLWithString:imageURL]
                         options:0
                        progress:^(NSUInteger receivedSize, long long expectedSize) {
                            
        }
                       completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished) {
                            /**
                                We can do this because the block "captures" the 
                                necessary variables in its context for reference 
                                when it starts execution.
                            **/
                           NSArray *visibleIndexPaths = [tableView indexPathsForVisibleRows];
                           if ([visibleIndexPaths containsObject:indexPath]) {
                               imageView.image = image;
                           }
        }];

        //... more code in the method

        return cell;

}