hello all 
i explain my situation
i have a NS-208 and upgrade OS
i have a mistake!!!
that is it.
i inject imagekey.cer file
CLI : save imagekey tftp imagekey.cer
and next time rebooting situation.
NS -208 has a boot failure symtomes like upper image
i try under method
1) retryed boot file tftp
-> still Invalid DSA signature, Bogus image....
what can...i...do....sad....
how can i solve this problem ?