If you see this message on the Manual Actions page, it means that some of your site’s images may be displaying differently on Google’s search results pages than they are when viewed on your site.
As a result, Google has applied a manual action to the affected portions of your site, which will affect how your site’s images are displayed in Google. Actions that affect your whole site are listed under Site-wide matches. Actions that affect only part of your site are listed under Partial matches.If you see this message, Google advises you to ensure that your site displays exactly the same images to users whether viewed directly on your site or within Google image search results. Google said this behavior may be caused by “anti-hotlinking” tools, and that may require looking through your site’s code on the server.
After you’ve remedied the problem, Google says to submit for reconsideration of your site, and to be patient.
“Watch for a message in your Webmaster Tools account — we’ll let you know when we’ve reviewed your site. If we determine your site is no longer in violation of our guidelines, we’ll revoke the manual action,” Google said in its help files.
Manual actions are penalties resulting from a human reviewer at Google who has determined a website violates Google's guidelines, and includes common spammy practices like user-generated spam, hidden text, unnatural links, and so on.
If your site is on the up and up, you’ll see the following in GWT's manual action viewer … and that’s a good thing: