I have the same issue, but with using CloudFront CDN (with a CNAME subdomain).
A week or so ago I saw google de-indexing my site's images in webmaster tools. Many of these images were named for better seo. At the same time the images were de-indexed I saw a big drop in traffic and rankings.
Of course, this may be a coincidence, but the site map for pages points to the origin image location, which is still valid, however when google indexes the web page all the images are pointing to a different URL (the sub domain for the CDN). I am wondering if there has been another google update and this mismatch has raised a flag in google that's given me a SERP penalty.... Which of course would be ironic if this is the case, given that this is an excellent SEO plugin.
I thought of using Yoast's API to set all image locations to one of my distribution servers, but that would still cause a delta between the sitemap and page image locations as Total Cache spreads the image path across a number of distribution server CNAMES. There doesn't appear to be an API in W3C Total Cache to get the image path it used and then inform Yoast's SEO plug-in.
Anyone else noticed an impact recently in their SERPS when there is a difference reported in the sitemap to what is actually on the webpage?