just a little heads up when you post personal pictures online whether you place them as avatars or in a post as a signature or part of a post it can be right clicked/copied \ saved to other peoples computers. this is just an action built into computers windows operating systems.
another heads up this site is searchable by google and other search engines which result in others who are not members are able to see what you post including your avatars, even with out registering as a member.
my point is short of you not using your personal photos\personal avatars there is no way to prevent members and non registered members around the world from using your personal pictures as signatures and avatars on any websites they choose to use them on.
for this reason I do not post any personal photos online anywhere, not psych central, not facebook not anywhere. this way I know my personal photos/personal information is where they belong in my home protected from anyone other than those I choose to share them with (family and friends)