When a publisher asks you for a book proposal with instructions to send copies of photo’s, how should I send those photo’s? They want hard copy sent through U.S. Postal service!
I’m not sure if I’m understanding your question correctly since the answer seems so easy to me. If your proposal is requested via hard copy and there are photos involved, then your photos should be sent via hard copy.
If the photos are an integral part of your book (it’s a photography book, for example) I would suggest sending actual prints rather than prints on standard paper so the publisher can get a better feel for the quality of the photo.