In Monster, if you keep going, every play-through will be different.
You can redesign any room with the Paint Roller icon if you want, but if you really want to clear images, the easiest way is to delete the app and reinstall, this will wipe all saved images.
Alternatively, you can go to your player profile in myOsmo, and reset your progress there. To do that, go to https://my.playosmo.com/, click on your name in the top right corner and select "Profiles" from the dropdown menu. Then select the player profile that you want to reset progress on.
If you have many players (such as in a school setting), we know this is not the most ideal situation, and we're working on adding in a manual reset that should come in a future version.