can we allow the user to choose one of his photos so that he can set it as wallpaper on his watch? I have not found any explanation on this subject.
You can not do that with Galaxy Watch Designer. You can if you were developing in Tizen Studio and there is a pre-installed watch face that allows that but it is beyond the capabilities of GWD.
What I suggest is that you go to the Vote -Feature Request and suggest that up there. When you do other can view or vote on it to show if there is a strong support
Samsung Developer Program
I thought the Galaxy active 2 has that feature wherein they can add an Image to the watch face . It is called My photo plus