Guidance on Moving Photos from Ext HD to iCloud

Currently I have all of my photos saved on an external hard drive. There are still some photos saved to the "system" library (which were not deleted after transferring photos to ext HD). When I plug the the ext hd into the computer, I obviously cannot sync to my iCloud account due to the folder with ALL of my photos not being in the "system" library. When I alt/option open the ext hd and alt/option to select the system library, that reverts me back to the "some photos" issue I mentioned. I need someone, much more intelligent than me to give me a step by step, easy to understand, elementary school type explanation of how to move ALL photos from external hard drive to iCloud. Please don't assume I know anything about computer electronics. Also, please be aware that the GB space on the ext hd is extremely large, hence the fact I began importing photos directly to the hd instead of my computer.

I look forward to the assistance