Only a few designers are allowed to developer and distribute Samsung Galaxy Themes. In order to do so you must be an accepted Themes Partner. the next recruitment period will be September 1, 2023 - August 31, 2025
For more information see this Themes developer site.
If you want to sell your images to a current Theme partner be prepared to have licensing agreements and proof that you are the original creator and so on.
Ron
Samsung Developer Relations