Most SEO plugins will do this for you but the meta tag you need to define a specific image is
<meta property="og:image" content="LINK TO YOUR IMAGE" />
Using php you could do this slightly more inteligently and use the featured image (if there is one) assigned to a page/post and a fallback image for instances where there isn't a featured image. That would look something like:
<?php $url = wp_get_attachment_image_url( get_post_thumbnail_id($post->ID), 'thumbnail' ); ?>
<meta property="og:image" content="
<?php if ( is_singular() and has_post_thumbnail( get_the_ID() ){
echo '<img src="<?php echo $url ?>"/>';
} else {
echo '<img src="LINK TO FALLBACK IMAGE"/>';
} ; ?>
" />
Please note, I've not tested the above code so it may need adjustment.