Okay, two changes...
First line, you can't have a space in the [NC, OR] bit... And you need another condition to exclude the no_hotlink image itself from the redirect, or else you get an infinite loop.
RewriteEngine On
RewriteCond %{HTTP_REFERER} !^http://(www.)?grieversanime.org/images/screencaps/ [NC]
RewriteCond %{HTTP_REFERER} !^http://griever.xepher.net/images/screencaps/ [NC]
RewriteCond %{REQUEST_URI} !no_hotlink.gif$
RewriteRule .(jpg|gif|png)$ http://griever.xepher.net/images/no_hotlink.gif [R,L]
It's already setup/tested in your public_html folder... I'm just posting this for edumacationalizing reasons. :-)
EDIT: Not fixed yet, there's another problem... Gimme another minute.
EDITEDIT: Got it now, revised code above. Removed the "OR" from the first condition. Which seems counter intuitive, but if you think hard about the logic, it works. We don't want to rewrite if any of those things match. So we really want the default "AND" on all three. That is "not #1 AND not #2 AND not #3" and only if all that works, THEN we do the rewrite.