best way, in my opinion, to back this up is to copy the iPhoto library, replacing the previous iPhoto library each time.

I doubt you can combine your multiple libraries into one, but even if you could, you would have so many duplicates of your pictures that it would be a real pain trying to sort it all out.

Even with Gigs of data, Time Machine should take a fair while to fill a Terrabyte drive as the backups that it does are incremental, after the first full one.

P