Allow/deny image hotlinking with .htaccess

Just add another condition before the RewriteRule for each domain you want to allow.

RewriteCond %{HTTP_REFERER} !friendlysite\.com [NC]  

(presumably you don't care if the request is via http or https or whatever, so you can leave that out to make it more generic)

RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www\.)?mydomain\.com [NC]
RewriteCond %{HTTP_REFERER} !^http://(www\.)?otherdomain\.com [NC]
RewriteRule \.(gif|jpe?g|js|css)$ - [F,NC,L]

Will work, as this says.

"Refererr is not nothing, and referer is not matching mydomain and referer is not matching otherdomain.

If it were the case that you were trying to do the opposite (blacklist a set of domains from hotlinking) you'd do something like

RewriteCond %{HTTP_REFERER} ^http://(www\.)?baddomain1\.com [NC,OR]
RewriteCond %{HTTP_REFERER} ^http://(www\.)?baddomain2\.com [NC]
RewriteRule \.(gif|jpe?g|js|css)$ - [F,NC,L]