1. From the main Stories screen, tap the pencil icon for the Story you would like to edit (or tap "+" to start a new Story).
2. From the Storyboard, tap the page that you would like to edit (or tap the "+" to select and add a new page).
3. Tap anywhere on the image/video area.
4. If replacing an existing image/video, tap "Replace". For new pages, skip to Step 5.
6. Tap the back arrow in the upper left of the window till you enter the root "Locations" screen.
7. Tap to select the source you'd like to import assets from.
Your source's app must be downloaded to your device in order for it to appear here.