How do I store my photos, outside iPhoto?

I am the type of person who always updates. So I am getting my iMac ready to upgrade to El Capitan and I am worried that they will soon completely phase out iPhoto. I am using the new Photo, but I did not transfer all my photos from iPhoto as they are kept on an external hard drive. I want to prepare for the possibility that I eventually will not be able to use iPhoto. So how do I get my photos out of iPhoto, and yet keep them on the external hard drive. Do I simply select them all and export to say a file on my external hard drive. This is assuming that in the near future, iPhoto will stop working with the release of the newest OS, which I feel is eminent. What is the easiest solution without importing them into Photo. My photos in Photo are stored on my internal HD at the moment. My iMac is a early 2009 running Yosemite.

I simply want to keep my iMac HD from filling up with photos, since I take a lot of them. I also don't have the money to spend on upgrading my iCloud storage or any other cloud type storage.