ScreenOS changed the signing key a couple years back. Your new image has the new key while you device has the old one. You are suppose to update the key just prior to the upgrade.
http://kb.juniper.net/InfoCenter/index?page=content&id=TSB16496
You can use this command to remove the old key and allow the boot.
delete crypto auth-key
Then once booted download the new signing key from the Juniper web site and install it here.
- Configuration > Update > ScreenOS/Keys
Select the Image Signature Key update radio button