It's in this background gif, and it's included in the game assets files. It is 72MB big so if you check the assets files it should be easy to find.
Edit: I did the research for you.. for this background it's this file: https://img.itch.zone/aW1nLzEyODE0NDUuZ2lm/original/TYouqL.gif
For the game assets, I don't know as I deleted the file, but if you open your game directory and order by file size you'll spot it easily :-)