I kept using the Photos app on the iPhone.
The trick is to choose a 4:5 (width:height) aspect ratio -- that's the same as the AW display. You don't have to make it any more complicated than that.
The
other trick in Photos is that if you try cropping a landscape photo and select one of the presets, it'll assume that you want to keep it in landscape, so it'll have a 5:4 preset and NOT the 4:5 you need. The workaround, then, is to first drag the crop border so it's taller than it is wide,
then press the Preset button in the bottom right corner. Then move, resize, and finish as usual.
I wrote this back in 2015:
https://forums.macrumors.com/threads/quick-n-dirty-watch-background-pic-tutorial.1919921/