Choosing desktop wallpaper from (my) Photos in macOS Sierra

That is a great idea. How are you testing for Landscape or Portrait in a smart album? Do you have a keyword set? I do not see a constraint to check the aspect ratio.

Now, it's with keywords. I think searching for aspect ratio worked back in iPhoto, and that's when I did most of the tagging: it was "Text includes Portrait" and "Text includes Landscape". (I've just discovered I do still have those folders in Photos, but they don't work properly anymore.) Occasionally now I add a "OrientationLandscape" or "OrientationPortrait" tag on, but as with so many things, I've abandoned what were once useful organizational practices because they're now a major hassle in Photos versus what they were in iPhoto.

At this point the main filter I do is pretty dumb. I have a smart folder that's simply "Photo is Favorite" and "Keyword is not Desktop", and then the ones that are in landscape orientation jump out clearly and it's easy to tag them with "Desktop". Then I use a "Keyword is Desktop" smart folder to assign wallpaper - but as I said, I don't think it's possible to assign wallpapers for different desktops from different folders in Photos. All six of my desktops use wallpaper chosen at random from the same folder (no, they don't choose the same picture each time, except for once-in-a-blue-moon coincidences).

Note that after I've done all that, it's also pretty easy to have a folder of favorites in portrait orientation, which is quite useful for iPhone wallpaper. But again - this is a lot easier because I did the orientation tagging for the bulk of my library back in iPhoto days.