Go to your Google search console and look at the index status. Mid March for some reason Yoast messed up and started to allow Google to index image posts. Since these image posts are only the image this is thin content which Google doesn't like. Combine that with older sites and it might trigger Panda or the version of Panda that lives now.
Edit: To give you an example, I had one site with a Google index of 100 that went to 900 within a week when looking at the Search Console. That is not natural and I don't have 900 posts. Those extra posts were Yoast allowing all the images on the site to get index. WordPress creates a post for all media such as images and since these posts are thin Yoast for years has been making them noindex but Mid-March something messed up and they got indexed. Another way to check is to go to Google and do the "site:YourDomain" search to see how many results you get. If the results you get are 1.5x more then the actual post you normally have then something is wrong. To fix it go into your WordPress, then to the Yoast Plugin, then "search appearance", and under the "media" tab make sure the button "Show Media in search results?" is set to "NO". This will keep Google from indexing your images.