[Plugin: NextGEN Gallery] Problem with Google button

I have a website : http://www.nicolas79.ch where I would like to share page on Google+. I would like to take the first image on the post. Now when I share a page on Google+ we have the header’s image. Maybe I would like to add <img itemprop=”image” src=”thumbnail.jpg” /> on singlepic. Can you help me please.

http://ww.wp.xz.cn/extend/plugins/nextgen-gallery/