The main issue with this method is that it's performing a task ([NSData dataWithContentsOfURL]) on the main thread and is therefore blocking ui actions (such as scrolling) until it has finished loading the image. A secondary issue is that this method doesn't dequeue sufficiently. Dequeuing is a critical factor in increasing a scroll-based class'…

gistfile1.m

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"MemeCell"];
    if (cell == nil)
    {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"MemeCell"];
    }
    
    // Dequeue subviews
    NSInteger tag = 1415;
    UIImageView *imageView = (UIImageView *)[cell viewWithTag:tag];
    if (!imageView)
    {
        imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 320, 320)];
        imageView.tag = tag;
        [cell addSubview:imageView];
    }
    
    NSInteger activityTag = 54321;
    if (!imageView.image && ![cell viewWithTag:activityTag])
    {
        UIActivityIndicatorView *activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
        activityIndicator.tag = activityTag;
        [cell addSubview:activityIndicator];
        activityIndicator.center = cell.center;
        [activityIndicator startAnimating];
        
        // No image set... fetch it on a background thread & set it dyanmically (weak reference).
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^(void)
                       {
                           NSURL *url = [NSURL URLWithString:[photo_urls objectAtIndex:indexPath.row]];
                           NSError *error = nil;
                           NSData *data = [NSData dataWithContentsOfURL:url options:NSDataReadingMappedIfSafe error:&error];
                           UIImage *image = [UIImage imageWithData:data];
                           imageView.image = image;
                           
                           [activityIndicator stopAnimating];
                           
                           if (error)
                           {
                               /**
                                 * To prevent -dataWithContentsOfUrl from being called on a faulty url again, we're going to
                                 * leave activityIndicator, which will block future attempts (dequeueing) to load the image.
                               */
                               
                               NSLog(@"Error loading image at url (%@) error: %@", url.absoluteString, error);
                           }
                           else
                           {
                               // Removing activityIndicator from it's superview will reset the cell's -viewWithTag: results.
                               // The imageView's image is set, we don't need to call -dataWithContentsOfURL: again.
                               [activityIndicator removeFromSuperview];
                           }
                       });
    }
    
    return cell;
}