A tip from hackwordpress.com to stop google indexing our images in wordpress. There are two simply ways to do this. The first adding this code to head tag:

<meta name="robots" content="noimageindex">

and the second method is adding code to our robot.txt in root wordpress directory.

User-agent: *
Disallow: /images/


or

User-agent: *
Disallow: /upload/

Via hackwordpress.com