After switching to a CDN I noticed that my indexed images went down. I have close to 1000 images on the site with correct tags, alt tags and all of that. Right now we have about 1000 images with only 50 actually being indexed in the organic search. Its a wordpress site with Yoast SEO plugin, using Maxcdn as my cdn.
Moving images to a CDN is similar to migrating the URLs of other content.
I helped move images to a CDN a few years ago. With those rules in place, we didn't lose any image search rankings.