When we got our 2 passport photos from the passport picture place, they already came in an envelope, so we just sent them in that and clearly labelled it.
Also I'm not sure exactly what you're asking about the 20 photos too - if you're saying you would put them in the same envelope as the passport photos, I think that might not be a good idea because if I remember correctly that would not be in order on the checklist. You could put your 20 photos in a different envelope though. I also saw someone say they got a binder and put the photos into sleeves for easy flipping.
Generally the method of presentation seems pretty flexible as long as everything arrives safely in one piece and in the order on the checklist.