Hi Reload,
Greetings,
I believe these logs, i.e. bogus image not authenticated are observed because the ScreenOS firmware is not successfully authenticated by the new image key during installation.
********Invalid DSA signature<- The installed boot loader (OS Loader) cannot be authenticated using the new image key********Bogus image - not authenticated
date = 9422, sw_version = 808031, cksum = 954806c3********Invalid image!!! ********Bogus image - not authenticated!!!
These logs have been reported through TSB.
Link: How to Update the New Image Authentication Key and Upgrade Boot Loader/ScreenOS Firmware
Hope this helps. 
Please mark "Accept as solution" if this answers your query.
Kudos are appreciated too!
Regards,
Sharat Ainapur