We do not have options for the blog pages to allow the setting of the facebook description and image as normally membership pages would not be shared on facebook like squeeze pages and sales letters do have.
There is a way you can manually set the values in the meantime:
In the Tracking codes field in the on page settings (inside the Page SEO & Optimization Options panel which you will find below the visual editor when you scroll down the page), in the header field box use these values:
<meta property="og:image" content="ADD YOUR IMAGE URL HERE"/>
<meta property="og:description" content="ADD YOUR PAGE DESCRIPTION HERE"/>
Once you have added these values and saved your page - run your page through the facebook debug tool to refresh their cache (temporary file on your site) and see if it shows the new values you set:
This should refresh the values on your site and if you specified a valid image and description you should see it on the debugger page and then this means when someone shares your page those images and descriptions will be used.
You can also use the following code in the same area to setup a specific title for your page when shared on Facebook:
<meta property="og:title" content="ADD YOUR TITLE TEXT"/>