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If you share a link on Google Plus it will often grab a fews images you can attach when you post the image. Facebook has a similar feature. You can tell Facebook which is the primary image for a page with a simple LINK tag. However, this isn't working for G+.
How do you tell G+ what the primary image for a page is?
Here's the information copied from the +1 button documentation (tweaked a bit for improved readability). This method applies to all ways that URLs are shared on to Google+.
Populating the +Snippet: After +1'ing a page, the user is given the option to share the page to Google+ via a displayed Share bubble. This share bubble (along with the resulting Google+ activity post) includes a preview, or +Snippet, that contains the page title, a brief description of the page, and a thumbnail image. These pieces of data are extracted from the target URL's content in one of four ways, listed in order of precedence:
Schema.org microdata (recommended): If the page is annotated with schema.org microdata, the +Snippet will use the name, image, and description properties found on any schema.org type.
<body itemscope itemtype="http://schema.org/Product">
<h1 itemprop="name">Shiny Trinket</h1>
<img itemprop="image" src="image-url"></img>
<p itemprop="description">Shiny trinkets are shiny.</p>
</body>
Open Graph protocol: If the page contains Open Graph properties for the title, image, and description, then they will be used for the +Snippet.
<meta property="og:title" content="..."/>
<meta property="og:image" content="..."/>
<meta property="og:description" content="..."/>
Meta "title" and "description" tags: If the page's <head> element contains <meta name="title" ... /> and <meta name="description" ... /> tags, the +Snippet will use their content attributes for the title and description, respectively. For the thumbnail image, the sharebox will attempt to find a suitable image on the page.
<meta name="title" content="..." />
<meta name="description" content="..." />
Best guess from page content (not recommended): If none of this data is present, then Google will parse the page and attempt to find the best title, description, and image.
[Sorry for the speculative response, but I don't have a site I can try this on immediately]
I've only seen information on doing this for the +1 button. It seems reasonable that on the Google+ back-end this is the same process and so would look for the same information. Assuming your templates are constructed in a way you can apply itemprop="image" to your desired image, give that a try.
Google now has a share snippet preview tool to generate appropriate schema or open graph markup: https://developers.google.com/+/web/snippet/
