Which do I use when moving photos from iPhoto library to ext. HD (preview, master or thumbnails)?

Hi all,

Sorry if this is already asked but I couldn't find anything...

I'm trying to clear up my (multiple) different back-ups and folders from the past 10 years, after many different apple laptops - particularly the photos. Essentially at the moment I have photos saved all over the place, I have 3 or 4 different iPhoto libraries with different photos, as well as other photo files on my external HD and in the old Time Machine backups. What I want is one folder, on my external hard drive, with all photos I've ever taken stored (I have a lot of spare time to sort through them all). I don't need (or want) to access these through Photos and plan to store them on an external disk.

My questions are:

When I go into the iPhoto libraries in Finder (by 'right' clicking on Photos Library > show package contents), there are a bunch of folders. The images themselves seem to be in Masters, Previews and Thumbnails. Which folder do I need to take the photos from that I want to move to my external HD? Am I correct that it's from the Masters folder as I never edited the photos, so I just want the originals? My searching on forums has ended up with me very confused...

Also, once I have these photos all exported and moved to my external hard drive in one big consolidated photo folder, I assume I can delete all these old 'Photos Library's' from my back ups of old Macs?

Thank you so much in advance :)

Lizzie