If you create your post that way, Day One will prompt you to change the new entry’s date from today to the date the photo was taken. Plus, as you add new photo entries, a top-level /Photos section displays them all in a slick, chronological photo collage.
Finally, if you have Camera+ installed, you can use it to take or edit the photos you add to Day One. At any step of the process—adding a photo, editing an existing entry’s photo—you have the option to switch to Camera+ to shoot a new photo or use its edit and filter tools to craft the perfect shot for your entry.
via Leo Marihart