A relationship (rel) attribute inserted into an HTML link to indicate the author of the page the link appears on. Google recognizes two formats.
A link tag inserted into the <head> of the page as follows:
<a rel="author" href="https://plus.google.com/012345678901234567890"/>
A link in the content of the page that points to the author’s Google+ profile:
<a href="https://plus.google.com/012345678901234567890/about?rel=author" target="_blank">Joe Author</a>
The number strings in the links above indicate the Google+ profile ID of the author.
Once Google has verified the link, it will include the author’s profile picture with the page listing in its results.
A Google search result displaying the Google+ profile picture of the author.