when you are planning to change the photos make sure that the app is opened on your watch ( the watch app should be active when you do the sync from the mobile)
on your watch open the app ( it will show a list of no photos or ur old photos if you already manage to upload them)
at the same time on your phone open the Fitbit app, select your Fitbit watch go to the gallery, and follow the steps in the attached images
go to your watch make sure to turn on Free Photo Scroller app ( it will show a list of no photos ) keep the app open will going through the following steps