For more details look at Micropub and WordPress: Custom Posting Applications. It uses the W3C Micropub recommended spec and most of the current clients are open source if anyone wants to build their own web or mobile interfaces.
Sunlit for iOS is a Micropub compatible app that supports photos. PhotoPostr looks promising too for photos and collections.
If you want to go crazy to support some of the other niceties on your site, add Simple Location plugin for showing GPS location and weather conditions and Post Kinds plugin for custom (and customizable) photo display (meant to be compatible with Micropub, but doesn’t have Gutenberg support).
Here’s an example photo from this morning: https://boffosocko.com/2020/09/09/55776473/
Naturally it’d be awesome to see Micropub support in core.