Where are the desktop pictures kept?

I placed two of my own photos in Macintosh HD > Users > [me] > Pictures, to use as my desktop pictures. In the System Preferences > Desktop & Screensaver > Desktop pane, in the scrolling list window on the left, at the very bottom, is a heading named Folders, and in that is the Pictures folder that I have my two pictures in. When I select this folder, the two pictures appear in the preview window to the right. When I select one of these pictures, it immediately appears on the computer's desktop. Fine. When I check the "Change Pictures" box, both of the photos appear in the rotation, but the default Sierra picture does too, though it doesn't appear in the preview window. How do I get it out of the rotation?

Second, in the scrolling list window on the left, at the very top is a folder named Apple, and in it are the Desktop Pictures and Solid Colors folders. In my Desktop Pictures preview window to the right there is another photo of mine that I had used as a desktop picture in the past. I'd like to add this to the rotation, but I can't find where it's stored. Searching on its name with the File > Find function in Finder does not locate it anywhere on my computer. I don't recall how it got into the Desktop Pictures folder in the first place, and just to hasten my descent into madness, File > Find does not find this folder either!

Can someone explain how pictures are identified for inclusion in the rotation and where they're located?