Been driving me crazy.
I have a client's site that is using a CDN that has multiple CDN subdomains.
Initially I used Yoast WordPress SEO as is and submitted to Google Webmasters and only about 500 images were indexed, so I looked at the Yoast WordPress API and changed it submited CDN url instead of actual WordPress url. The problem is the code from the API doesn't have any examples if you have multiple subdomains. So I set it to 0.design-milk.com and now two weeks later over 4k images have been indexed, but still nothing on the other 55k images. Any ideas?