You can pick a picture for a position in multiple ways.
Select the position by clicking it in the collage.
Use the pick a picture button in the bottom panel of the application.
Drag the picture from Photos app to a new Folder in Finder.
Select the position by clicking it in the collage.
Use the pick a picture button in the bottom panel of the application.
Select the picture from Finder.
You would only be able to select a position if pictures are unlocked at the top of the application.