originally, the size of the pictures were restricted by facebook, actually, to 110px max.

this is set in file /components/com_muscol/views/album/view.html.php around file 202 you see this:

$document->addHeadLink( $uri->base() . MusColHelper::getThumbnailSrc( $album->image, '110', '110' ) , 'image_src');

you can change the size to say, 300, like this:

$document->addHeadLink( $uri->base() . MusColHelper::getThumbnailSrc( $album->image, '300', '300' ) , 'image_src');

yes, you can add this, in the same place:

$document->addCustomTag( '<meta property="og:image" content="'.$uri->base() . "images/albums/". $album->image.'" />' ) ;