Adds a photo as the listing's last photo. A listing's first photo is
its cover. Send the file as multipart/form-data (a photo part), or
a JSON body with the url to download it from.

• PNG or JPEG only, judged from the file itself, up to 10 MB, and at
  least 1024×683 either way round (683×1024 is fine). Airbnb, VRBO and
  Booking.com all take a photo like that.
• url must be an https link to the public internet. It may redirect
  up to 3 times, and the download must finish within 20 seconds.
• A listing can have at most 50 photos. With 50 or more, delete some
  first; reordering, captions and deleting still work.

The photo is stored in Host Tools and shows on direct booking sites
straight away. While the listing's content sync is on, it also goes to
Airbnb (see "Content sync" on /api/setListing/{listingID}). It isn't
sent to VRBO, Booking.com or Houfy yet.