I still use IRC. There are now modern web clients like The Lounge or Convos that can display/share images in the channels, keep history and push notifications. Apparently Convos can do video chat but I never tried it. Unfortunately I’m not aware of screen sharing features for any of these.

So on a very simple setup, you need an IRC server, then install and connect one of those clients to your server, and use them through a web browser, either on a computer or on a phone.

It’s obviously not entirely Discord-like, but it is a simple way to chat and share images.