I hope you have new image and imagekey.cer downloaded from the website, not any old downloaded software.
1: Reboot only after upgrading the imagekey.cer. and then image and then reboot. updating the imagekey.cer only and rebooting the device will cause such issue.
2: Image might have been corrupted.
Either case, you need to upgrade the image from using TFTP server. Set an TFTP server ( 3c demon, solarwind etc) and connect to the Eth0/0 and follow the KB http://kb.juniper.net/InfoCenter/index?page=content&id=KB5519&actp=search from the step 5. Try downloading new file from the site to avoid any image corruption in the old one.
Thanks,
VIkas
Thanks,
Vikas