I had the same question when building our first site, and wrote one of the sponsors and asked about how I would either obtain a license, or how to show legal use in the 2257. They replied that in the terms of use when you first signup with the sponsor it covers giving you a blanket "permission" to use the images to promote their sites, and that I didn't need to "do anything". I still wonder about that answer, and curious to see what others have to say about it.