You just need to add authorised domains to the list of referrers...
RewriteCond %{HTTP_REFERER} !^http://www.yoursite.com/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://yoursite.com*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://yoursite.com/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://www.yoursite.com*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://yoursite.com:80/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://www.yoursite.com:80/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://yoursite.com:80.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://www.yoursite.com:80.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://www.othersite.com/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://othersite.com*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://othersite.com/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://www.othersite.com*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://othersite.com:80/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://www.othersite.com:80/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://othersite.com:80.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://www.othersite.com:80.*$ [NC]
and then place the ONE image into a folder on it's own with the .htaccess file.
Note that I didn't include the full .htaccess code, just the referrers bit.