Before you start, ensure you have ticked 'Hidden Items' located under the 'View' tab in File Explorer.
After that, simply navigate to your 'AppData' folder and locate the 'Koan' folder. Inside will be a folder called 'SketchCrawler.' Delete the 'SketchCrawler' folder. Next time you start your game, your custom artwork should be gone.

Folder location:*

C:\Users\YOURNAME\AppData\LocalLow\Koan\SketchCrawler

*Location may vary for some users.

NOTE that your current save WILL also be deleted if you delete that folder.