First, check to see if the "undo" button in the top left of the Lightbox is available, that's the quickest way to recover from deletion if it was the last action.
If that doesn't work tap the disclosure triangle next to Lightbox and in the new menu that pops, under the Lightbox entry, there is a "Deleted" option. Select this and you will see the thumbnails of photos that have been deleted but not yet removed from your device. Tapping on each thumbnail will allow you to recover or remove each photo permanently. Tapping "Select" in the top right will allow you to recover or remove either all or a selection of these photos.
Please note that if you have deleted the entire application the only way to recover your photos is by restoring your device from a backup.