Could you please leave your forum  URL to allow us to check it?

function my_default_og_image( $image_url, $type ){
if( $type == 'og:image' && !$image_url){
return 'https://example.com/images/mylogo.jpg';
}
else{
return $image_url;
}
}

add_filter('wpforo_find_image_url', 'my_default_og_image', 10, 2);

I entered this into my child theme (after changing the image example to my own) and it doesn't work.  Any ideas?

Is there a way to share a specific image from a post to Twitter/Facebook, or only the default option above?

For non-topic (forum home, forums and other) pages and for topics without an image wpForo allows to set default share image.  That's the default share image (og:image share tag) . You should put this code in WordPress theme functions.php file to set it. Just change the red marked image URL to your website/forum logo URL or to something else, it'll be the default share image:

function my_default_og_image( $image_url, $type ){
    if( $type == 'og:image' && !$image_url){
        return 'https://example.com/images/mylogo.jpg';
    }
    else{
        return $image_url;
    }
}

add_filter('wpforo_find_image_url', 'my_default_og_image', 10, 2);

How to Easily Add Custom Code in WordPress (without Breaking Your Site)