Not sure if the board is doing this, or if it's the way you have it in your .htaccess...
make sure this is all on one line
RewriteRule .*\.(gif|jpg|jpeg|bmp)$
http://www.yourdomain.com/images/hotlink.gif [R,NC]
Plus, does the image you are now substituting have a "jpeg" or "jpg" extension? I noticed in your example, it shows "jpeg".
If you normally use this file extension, then you will also want to be sure to add "jpeg" into the code, as shown by my example. Otherwise, these files will be free to be hotlinked at will.