If you are using the current app on the app store there is no way to bring back any drawings or screenshots. There is no save and load functionality. You can only do screenshots and share them.
In the new Tagtool Beta you are able to save your drawings to the deck and you can also load in pictures from your photo library. You can also do a screenshot of your current work with or without transparent background and load it back into the scene.
If you are not a beta-tester yet, just follow this link to the registration.
Happy about any feedback!