Hello, our website has millions of images. Prior to 10/9 our image index rate according to the GWT sitemap report was good and steady. Then suddenly around 10/9 our number of indexed images took steep drop and has been slowly but steadily declining ever since. This is a mystery to us. Here are some thoughts / info:

1) Our image sitemaps look fine, and returns no errors.
2) The web PAGES in the sitemap are indexing just fine...only indexed IMAGES are dropping.
3) Several months ago we suffered an indexing disaster thanks to an "seo expert" screwing up canonical tags...this was pretty quickly fixed...but perhaps we are now seeing delayed impact from it in GWT?
4) A "Fetch as Googlebot" and several other page analysis tools don't report any on-page issues...as far as we can tell our images and image URLs are fully accessible by spiders.
5) Despite the GWT indexing report showing a drop, doing a "site:" search for our domain in Google Images looked great... until last week...now it is also returning less indexed images.
6) We are using a CDN to deliver content, including images, however this has been in place since 2006 so it is unlikely it would only now be a problem...unless something changed on Google's end, like an algorithm.

Any ideas on what might be happening here and how to reverse this trend are greatly appreciated! Thanks!