Additional .htaccess rules based on wp page

I’m pretty sure you have wrong “rewrite” parameter in your add_rewrite_rule call.

I’m writing it directly in here, so it can be buggy… But it should look something like this:

add_rewrite_rule('^authors/([^/]*)/([^/]*)/?','index.php?page_id=<YOUR AUTHOR PAGE ID>&my_author_name=$matches[1]&my_author_pic=$matches[2]','top');

Then you can use add_rewrite_tag (http://codex.wordpress.org/Rewrite_API/add_rewrite_tag) to add your custom query variables (my_author_name and my_author_pic).

And in your author page you should use these custom query variables.