Upgrade from 6.3.0r15a.0 to 6.3.0R22 doesn't need any intermediate upgrade, you directly can upgrade to the 6.3.0R22.
If you are getting **bogus image***** , image uthentication failed then please check KB https://kb.juniper.net/InfoCenter/index?page=content&id=KB5280&smlogin=true&actp=search .
Or else please let us know where/hot it's failing .
Thanks,
Vikas