I think your problem is that you're loading the image every time the cell will be shown, remember, you're reusing cell views.
I've used https://github.com/rs/SDWebImage it has a method to cache the image and show a placeholder.
[cell.imageView setImageWithURL:[NSURL URLWithString:@"http://www.domain.com/path/to/image.jpg"]
placeholderImage:[UIImage imageNamed:@"placeholder.png"]];