We are trying to share a post on Google+. As you know, as soon as you enter a URL, Google scouts the page and adds an image to it. There's an image on the page we would like to have associated to the post, but unfortunately it picks the wrong one. We noticed that it actually picks a set and you can scroll between the possible ones, but the image we want is not in this set. It is, however, on the webpage we are posting about with no fancy tricks, and it actually comes earlier than the other ones.
The first idea was size, but our target image is not particularly unusual.
How does Google decide which images to pick, so that we can adapt to that?