Your Sketchbook art files are .tiff files. These files contain the data for your layered art and are stored in the folder Internal Storage > Pictures > Sketchbook Gallery.
The PNG files you see in the subfolder Sketchbook Previews are thumbnail previews for the Sketchbook Gallery. They are not your art files and there is no value to Sketchbook in backing them up. If you ever need to restore your .tiff files in Sketchbook, new previews will be created and these files will be unneeded duplicates.
How to back up your art files on Android
- Navigate to Files or Internal Storage (this may be called something different depending on your device manufacturer)
- Find the folder Pictures
- Click in to the folder Sketchbook Gallery
- In this folder you will see a number of files with alphanumeric file names and the extension .tiff. These are your Sketchbook art files.
- Copy these files (do not delete, remove, or rename them) to a second location in the cloud or on an SD card.
If you ever need to restore a backup, you can copy these files back to your device and run the Recover Images tool by going to Sketchbook's main menu, and tapping Preferences. Scroll to the bottom of the screen and tap "Check for recoverable images".