Wallpaper didn't allowed me to manipulate the pictures at all (it updated to 13.2 so that may have helped), until i chose one specific image and tried to manipulate it. I had originally chosen to rotate through all the images without ever having chosen one specific image. Once I chose an image, I was then shown the image name and the toggle between fill/fit/stretch/center was then activated.
Before it just said "picture" as a generic place holder with a static "fill/fit/stretch/center" box that wouldn't drop down. For those of us who choose to randomly rotate through ALL the pics it might help to have a separate selection of how to modify the desktop pic when choosing to all vs. one or just a notice to choose one first and then select all. It eventually uses that selection for the one image as a default for all of them as a collection to rotate through.
"Modify the desktop picture’s arrangement or dimensions: choose an option from the pop-up menu at the top of Wallpaper settings. Options include Fill Screen, Fit to Screen, Stretch to Fill Screen, Center, and more."
