thulinma's tool is GREAT
the easiest way is to:
1. make your own design in-game which does NOT have to be pro. it can be a random scribble, as long as YOU make it. a regular one is simpler to convert. save it to the SD card, upload onto a computer as normal, stick it on your desktop.
2. on the website, scroll down to "Load ACNL file or QR-image:" and upload the 'design' you just made. it'll display the design and the name of it, the character who made it and what town they're from, along with some numbers. click "Copy Creator".
3. Now you can upload the QR you want to change. they can be any user-created code, as long as its clothes and/or pattern (like you put on the ground). im not sure about faceboards. if you want to upload a shirt or smth, upload the FIRST of the four patterns. itll then prompt you to upload the second, then third, then fourth. THEY HAVE TO BE IN THE CORRECT ORDER.
4. you can then "Paste creator" and your character/town info will be there in the same QR pattern. the QR codes below will change and now you just go use the sewing machine to scan it in as normal. the difference is that you can display these in the able sister's store because the game thinks YOU made it and you can edit it in the game.
the creator of this tool has said that she only wants you to use it FOR YOURSELF and to NOT claim others' designs as your own.
as to WHAT images to use, i'd say ones that are the default, like this:
Source
default being the normal able sister's background with sable in the corner. i know some people will make their ownbackgrounds but i think most people keep it the default. i'm not sure if the 32x32 images will work but youre welcome to try!!
sorry if this isn;t what you meant, haha aha.