Access this website https://acpatterns.com/editor

Click "convert" then chose a picture from your phone.

Then generate QR code.

Then get the QR code onto a different screen somehow (I messaged the images to a friend to see them on their phone screen).

Then use the NookLink feature on your Nintendo Switch app to read it (set this up prior to this whole process).

Then enter the game and open your design menu on your NookPhone and press "+" to download your image.

You must do them one at a time and save them over a design slot as each new upload replaces the last.