Not sure if this is already in the forum, did a quick search and didn't find it.
When someone shares your base url link on Facebook you can choose what image facebook will use by adding this to your theme's main.php
<meta property="og:image" content="http://www.yousite.com/oc-content/themes/youtheme/images/logo.jpg" />
Facebook scrapes pages every 24hs so you won't see a change immediately but can see it on:
https://developers.facebook.com/tools/debug by entering your URL
If in Object Properties/og:image (result from fb debugger tool) you see the image you want job done, it will be working soon
Facebook does a good job picking the right thumbnail to share in item.php so no need to modify that.
Extras: if you want to change or add a longer text for your site title/description that the one you specified in the admin panel options, you can go for the following metas for fb
<meta property="og:title" content="Whatever title" />
<meta property="og:description" content="description of your site"/>
EXAMPLE FOR ITEM.PHP<head prefix="og: http://ogp.me/ns# fb: http://ogp.me/ns/fb# website: http://ogp.me/ns/website#">
<meta property="fb:app_id" content="YOUR FB APP ID HERE" />
<meta property="og:type" content="website" />
<meta property="og:url" content="<?php echo osc_item_url() ; ?>" />
<meta property="og:title" content="<?php echo osc_item_title(); ?>" />
<meta property="og:image" content="<?php echo osc_resource_url(); ?>" />
EXAMPLE FOR MAIN.PHP<head prefix="og: http://ogp.me/ns# fb: http://ogp.me/ns/fb# website: http://ogp.me/ns/website#">
<meta property="fb:app_id" content="YOUR FB APP ID" />
<meta property="og:type" content="website" />
<meta property="og:url" content="http://www.yoursite.com/" />
<meta property="og:title" content="what ever you like, can be different from general settings in your admin panel" />
<meta property="og:description" content="what ever you like"/>
<meta property="og:image" content="http://your_logo_at_least_200x200.jpg" />
hope it helps someone. cheers!