The client was using an image host which supported image transforms. They uploaded all their images there and were using a lot of image transforms from that host on their website. So, HOST/img/123?scale=1.3&color=greyscale type business. They were paying per image loaded from the site.

We tucked cloudflare in front of the image host using a URL rewrite rule. Cloudflare's cache reduced the number of requests made to the image host by 99% or something.