Check out this cool tool:
http://www.htaccesstools.com/hotlink-protection/
It will allow you to add domains that you will allow to hotlink from you, and block all others. For instance, if you want to allow yourself to link between your domains, add them into the Allowed Domains box.. it will allow you to hotlink from X domain to Y domain with no problem, but anyone else trying to hotlink will be blocked.