Photo card (attempt)

hi John, sorry if this is bad form, but i'm not sure how to populate some of these variables. i know image_url should be: "https://data.ukiyo-e.org/ORG/images/ID.jpg", but this is probably in some variable as well as width and height? i'm happy to send a new pull request if you can point me in the right direction. More info on photo-cards at https://dev.twitter.com/docs/cards/types/photo-card