I think rather then doing the rewriting in .htaccess you should add new custom rewrite rule wordpress way.
function my_custom_rewrites(){
add_rewrite_rule(
'img/(.*)/comment/(.*)',
'index.php?image=$matches[1]&comms=$matches[2]',
'top' );
}
add_action( 'init', 'my_custom_rewrites' );
You can also try the tips shared here http://docs.dev4press.com/tutorial/practical/debug-wordpress-rewrite-rules-matching/ that might help you understand which Rewrite rule is matching the url and why a particular template is being rendered.
Your URL might very well be matching with other patterns of rewrite rules of Wordpress. So registering your own rule in wordpress gives you the opportunity to get it checked before the core rules, Notice the third parameter of the add_rewrite_rule which says 'top', this specify the same