What is <link rel="image_src">

Today I came across a <link rel="image_src"> tag. I don't know about it, so I use google. Google tell me that this tag are similar to og:image. So I came to open graph main site to read about it http://ogp.me/, but i found nothing about link rel="image_src". So this tag is replacement to meta property="og:image" or is in special tag in another specification ? How use this tag or for what is used?

The rel attribute specifies the type of the link, i.e. the kind of the relationship between the document and the linked resource. Usually just a few keywords, like stylesheet and icon, are used. Although many other keywords have been proposed and registered, most of them are write-only: they are meant to express something, but nobody cares (no software uses the information).

The extension mechanisms of HTML5 include, in the description of link types, a somewhat obscure mechanism that allows, in theory, anyone register his favorite keyword in the existing rel values wiki to make documents using it as rel value “conforming”.

And image_src has indeed been registered there, with the information that it is used to “specify a Webpage Icon for use by Facebook, Yahoo, Digg, etc.”, no specification has been identified but an article about it is linked to, and it is “probably redundant with rel=icon”.

You can use this tag to use an image as the thumb for link share.

When someone posts a link to your site on social media, such as Facebook, the image that is displayed with your link is usually the first one in your code. This may not be the image that best fits defines your site, and it may not fit well in the small box that Facebook posts. The link rel="image_src" tag lets you control what image (or images, you can have more than one by stacking separate references) is displayed alongside your link.