RewriteCond %{HTTP_REFERER} !^$ adds a condition for the RewriteRule that the referer header must not be empty. If you go to an image directly, then the header will be empty, so this condition fails and the RewriteRule is not effected.

Most do out of the box, but can be configured not to, in which case this solution will be ineffective, and will in fact prevent these users from seeing any images on your site.
If the browser were configured to not send headers, is it treated as an empty referer (and falls under the condition above) or something else?