You can only do it for images that the activation server is willing to keep singing for you. Sometimes they go back one or two versions (or a version is know to

You can only do it for images that the activation server is willing to keep singing for you. Sometimes they go back one or two versions (or a version is know to be jailbreakable, they won't allow it to sign). The funny part is that the signature is always constant for your device for each version so if you proxy your signing request and cache it (like use Cydia's "on file" thing) you can install and active for older builds.