This repo contains a script that can be passed to sips (man sips) to add rounded corners to a rectangular image, and then pad it with transparent background into a square image.
I wrote it to convert the card images stored on my Mac by Apple Pay into images suitable for using as item images in 1Password.
To run the export, first install jq if you don't have it already (either brew install jq or follow the instructions here), then:
./export-all-card-imagesThen check your desktop.