What does this actually mean:
I then downloaded the settings from my old Mac via the backup external hard drive.
You restored from back up. Did you overwrite the new iPhoto Library with the new Photos? How many iPhoto Libraries are in your Pictures Folder?
What I mean is that I used Migration Assistant to import all of my old settings and files from backup. I had thought that the new photos would be incorporated with the old. There is one iPhoto library in my Pictures folder.