I am using the Flickr API to retrieve a list of photos with associated sizes, returning as links to the raw image. These links are all HTTPS, as is the domain they are served from.
However, despite the document and all local resources being served over HTTP/2, all images from Flickr are served over HTTP/1.1. What necessary steps are involved in serving all resources over HTTP/2 where available?
I might be misunderstanding its implementation with Cross-Origin - or Flickr's API implementation - but as far as I can tell resources across domains should be able to use the HTTP/2 protocol where available server-side. Any reasons to the contrary or explanations why not would be greatly appreciated, as the newer protocol significantly speeds up the site in question, but serving them locally would take up much space.