if your sure that its nothing to do with the hotlink protection it sounds like a mod_security problem with you being redirected to the home page when trying to view the image directly. I'm sure someone will be along shortly to confirm this but if you want to give the mod_security solution a go just search the forum for "mod_security" and you will see the solution plently of times.
If that doesn't work then remove the mod_security solution as your account is more at risk with it in place.
