It's not a virus. It's malware that IM's you what appears to be a jpeg. If you attempt to open it, even logged in as admin, you will be prompted for a password because it is really a script. If you are not running as admin, then you will have to give it an admin name and password or your keychain password. If you are stupid enough to give an admin password to open what appears to be a picture, then it writes itself into iChat and tries to pass itself on to your IM buddies.