hey!! what you want to do first is open up ur “Photos” on ur apple iphone, and pick the picture you want, then in the bottom left there’s a share button, press on that and scroll down until you see “Create Watch Face”. Then press the top option saying “Photos Watch Face”, you can then customise it however you would like (it gives you options such as putting the time on the top or bottom) after that you should see an “ADD” button somewhere near the top right of ur screen press that, then open ur watch and it should be changed
hope this helps <33