There’s nothing better in Animal Crossing than strutting around with custom designs and this website will let you upload an image and then convert it into a Animal Crossing: New Horizons custom design. You can find the website right HERE.
It’s an instant process that churns out a QR code. You’ll need to then use the Nintendo Switch Online app on your phone, and also turn on NookLink in the main menu of Animal Crossing: New Horizons (on your Switch), before you can scan the QR codes with your phone (and have them show up in your game). You also should have the Custom Pro designer unlocked (which you can get from Nook Services).
If you don’t want to convert images, you can also just draw your own pixel art (the same way that you would in game). There’s also a library of creations that other people have created as well.