All images, attachments are stored in your /wp-content/uploads directories. Therefore Google and most bots just go directly to the source and bypass your page. For indexing just images, Google does not want your web page content, just the image. This is very common. I think you should enable these directories.

Can you disallow viewing the directory index but allow direct access to the contents, if they know they are there? It is easy for a bot to scrape your page and extract only the images/download files. This is a common script request. They then have the direct URLs. Google and others do this.

Google does not want your web page content, just the image

Rather amazing statement if you think about it. The images are available via the normal content, so it does beg the question why the need to bypass?

The images are available via the normal content, so it does beg the question why the need to bypass?

So they can cache a copy and wrap ads around it* rather than send the searcher to the site.
If they don't do it now, they will, this is "just in case we need them"..

You can stop the crawling with Disallow: in robots.txt for the folders you don't want crawled. They may still list these as "blocked resources" but they won't crawl if disallowed.

If you want them to have access to the images, you can use "Disallow:" for the folders and "Allow:" for the */*.jpg, */*.png or the filenames you use within those disallowed folders. Use Allow after Disallow.

Google will follow your instructions, but other search engines may follow or not. It doesn't block the robots, just tells them your preferences.