Your array contains strings not images. Also, it looks like you're trying to get the image view from self.view
when you should getting it from the cell
. Do like this:
UIImageView *flagImageView = (UIImageView *)[cell viewWithTag:1];
NSString *imageName = _flagsArray[indexPath.row];
UIImage *flagImage = [UIImage imageNamed:imageName];
flagImageView.image = flagImage;
Or more succinctly:
UIImageView *flagImageView = (UIImageView *)[cell viewWithTag:1];
flagImageView.image = [UIImage imageNamed:_flagsArray[indexPath.row]];