In addition to adding stories and editing photos, you can also create captions for your pictures in both entry and album modes.
Adding photo captions to entries:
From either the draft or edit an entry screen, click the speech balloon below the photo you'd like to caption. From there, a dialog box will appear. Add your caption here and click "OK" when you're done. Captions have a limit of 1000 characters.
Although photo captions don't appear on drafts they will appear on the published entries, like so:
Adding photo captions to albums:
From either the draft or edit an album screen, write your caption in the text box next to the corresponding photo. Remember to save and publish when you're done!