Q: how to change wallpaper or screensaver for watch

The Apple Watch has neither wall paper nor a screensaver so there's nothing to change.

Screen savers, designed to prevent burn in on cathode ray monitors serve little purpose on today's modern screens and would be kind of pointless on something on which the screen only stays on for 10 seconds.

Go to your photo app and select your photo. When it appears on your watch face, press and hold it. The watch face will appear over the photo.

The watch face will appear over the photo.

Good idea. Only,  it will be the very plain "Photo" watch face, where you can customize nothing but the photo and not add any complications.