Here is what I did, and it worked (mostly) to back up my photos to my unlimited photos with Amazon Prime:
(1) I made a symbolic link to the directory (folder) containing the originals inside my iPhoto library. This is normally hidden in the Finder, so I typed this in terminal:
ln -s ~/Pictures/iPhoto\ Library/Masters/ ~/Pictures/iPhoto-Masters
"ln" is the command to make a link; "-s" is the option to make it symbolic. "~/Pictures/iPhoto\ Library/Masters/" refers to the folder inside the iPhoto library where all the original are kepts. "~/Pictures/iPhoto-Masters" is the location and name I gave for the link.
After doing this, in my Pictures folder I can see a folder alias called "iPhoto-Masters". Inside that is a series of folders that are named after years.
(2) Using Google Chrome, I logged into my Amazon Cloud Drive. When I got to the page where I can upload photos (https://www.amazon.com/gp/photos/list), In the Finder, I dragged the iPhoto-Masters folder over the Chrome window, and it started uploading.
It failed on a few files (I have a few videos longer than the 2GB limit), and eventually used up all of my limit of non-photo files, but it uploaded most of my 271GB of photos, 8,710 photos and counting so far.
This of course does not back up any of the organization (albums, smart albums, slideshows, books, etc.) or changes I have made over the years in iPhoto. It does however preserve the original pictures.
I tried the same thing using the Amazon desktop app, and it didn't work. I assume that the browser's traversal of the file hierarchy gets around Amazon's limitation on uploading package contents.