A24 will reach out to me from time to time to help them with production design work.
My contact sends me a spec sheet, which has all of the format requirements of the various digital outlets the artwork is to appear in (iTunes, Google Play, Comcast, Amazon, etc.), I will get briefed in on any compositional requests as to how the art should be treated, and then I just go to work reformatting the key art into all the required dimensions while maintaining resolution and saving for web—shooting for file sizes around 100K and delivering on a tight deadline.
Films I have worked on include A Ghost Story, Menashe, Good Time, and Into the Forest.