The problem is you're using the image to promote a sale. That's commercial use. I realize you're doing this innocently, and odds are good DC's lawyers won't notice and will only give you a warning if they do, but it's problematic—it certainly does not fall under fair use, because the use is purely promotional. I'm not a lawyer, but I've been a professional writer for over 30 years, so I've had to learn more about trademark and copyright than most people.