this assumes null referrers and domain.com and anotherdomain.com are domains that you WANT to be able to hotlink from. If you want to block null referrers just remove the top line.
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www\.)?domain.com.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://(www\.)?anotherdomain.com.*$ [NC]
RewriteRule \.(jpe?g|gif|bmp|png)$ - [F,NC,L]
Any request for an image from anywhere else will be given a 403 response
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