If you accidentally rejected (swiped to the left) a photo while choosing images in the Photo Judge, you can drag the image back into the center from the rejected (red, left-hand side) column and into the accepted (green, right-hand side) column.
If you deleted a photo from the Photo Detail View, you can add the photo back by using the Add Photos feature ("+") from the Story Day View to select the desired photo (or photos) from your camera roll.