I don't have a specific date, but Google has changed its crawling method. I think Google is now ignoring the "static" version (before the javascript runs) and just evaluates the "final" (rendered) version.
For articles, I embed resized images that link directly to full-size versions. When the DOM is ready, javascript replaces those links with links to optimized gallery pages. Around July-August, full version images started dropping from Google Images results.
Funny thing is, some of them are now appearing under a different website that iframes my pages with sandbox (javascript never runs, so Google crawls full-size image links)
I've switched from replacing links to attaching window.open events and Google started to crawl & include full-size versions.