With the following code I can replace the hotlinked image with another image. However, is it possible to force download the replaced image instead of opening in the browser?
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http(s)?://(subdomain\.)?mydomain.com.*$ [NC]
RewriteRule \.(jpg|jpeg|png|gif)$ http://i.imgur.com/D5ZIJBq.jpg [NC,R,L]