As part of your migration from StreamElements, you might want to download the art files you previously used in StreamElements so you can create new products in your Fourthwall shop.
To download the art files, navigate to the merch tab of your StreamElements dashboard, and you will see a button on the top left corner that says "Assets Gallery." Click there to open your asset library with all the art files. You can click each art file so they download to your computer drive.
Please note that this flow only works if you have connected your Fourthwall shop to your StreamElements and only art files that you used in StreamElements will show up in that Asset Library.