Luckybackup can do what you’re talking about. It uses rsync, and has cron options to schedule backups. Personally I prefer a cloud solution, specially after reading a story where someone did have proper backups, but stored in his house which burnt down, including computers and backups. But cloud storage comes at a price.
> But could we have a external HDD and keep Pictures from the PC mirrored
> to the external, in an uncomplicated manner?
Sure.
Just mount that external disk always in the same place, then you can
copy paste the same backup command, or use a script. I would use rsync.
If he has two computers, he could have the photos on two computer, and
work on any one, using unison. That’s what I do. Unison decides at run
time which file is newer on each side, and copy to the other one, or ask.
In your case it is always the same direction, so rsync is perfect. Or
any variant of it.