You can add a text caption to any picture by tapping the small icon in the bottom left corner of the caption window under any picture.
Note for Power Users:
A text caption is simply a text page linked to a picture. You can easily:
- De-link it and move it around the story
- Turn any text page you've already created into a caption
- Move it to below (or above, if you prefer) any photo or video
- Link it to the photo or video