.htaccess hotlinking but allowing multiple sites??

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Hi there.. please excuse me if this is a very noobish question.

I know (from searching the web) that I can prevent people linking to my
images using

RewriteEngine on
RewriteCond % !^$
RewriteCond % !^http://(www\.)?mydomain.com/.*$ [NC]
RewriteRule \.(gif|jpg)$ http://www.mydomain.com/angryman.gif[R,L]

and replace content with well.. something different. ;)

but what I'm really wanting to be able to do is limit access of images
to one or more sites/ referers?

for example. I might have image.gif on my own website.. but I myself my
want a link on another site (a forum perhaps) .. what I'd like to do is
allow the image to be shown on my own page AND on the forum site but
nowhere else (obviously I'd have to manually maintain the list of
allowed referrers / users /whatever you'd call them)

I wouldn't want my images etc to be indexed by web spiders.. is such a
thing possible with a single .htaccess file? or am I into the realms of
something more complex?

please excuse me if it's a simple solution.

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