A bug that appeared in versions 7.0 and 7.0.2 of the popular WordPress SEO plugin Yoast, caused each image on the page to load as a separate page instead of loading as a file leading Google to index it as an intirely new page. Since these pages contain only an image and no other content, Google's algorithm concluded they were low-quality.
On sites where many pages like this have been created, other pages have also downgraded due to different Google algorithms (especially those related to content quality, such as Panda), that sometimes work across the entire site. Among sites that were severely damaged there were sites that get a lot of traffic from Google image results
What to do if your site has been damaged?
Check your settings: If Yoast is installed in your site, you should have an SEO item on the sidebar. Enter “Search Appearance” and then “Media”. The first option should be "Media and Attachment URLs". Make sure it’s marked "Yes". If not, check “Yes” and save settings.
Although this bug has already been fixed, this might be a good opportunity to mention that if you have other critical extensions responsible for key features of your site, you should always read the comments for each new version of each plugin (located near the plugin update link in the plugins menu).