You can attach a photo to the desired activity by clicking on the Activity and tapping “Edit” on the top right of the screen. From there, select “Tap to add a photo” to include either a new or existing photo.
When your selected Activity is due, your clock (or “O”) will display your Activity Photo in the center. It will remain there for the duration of your Activity. This is great for motivational content around exercise or Nutrition! Your Activity photos are also reflected on the Apple Watch.