As long as they're hosted somewhere on the internet, you can add images with regular Markdown. Built-in support for photo management is in the pipeline though!

Is Snap.as open source too?

No, currently - Snap.as ( https://snap.as/ ) doesn't seem to be open-source and is only available (as a service) if you have a paid Write.as account ( https://write.as/pricing ).

I tried looking at the Snap.as repo ( https://github.com/snapas ) and Snap Freely organization ( https://github.com/snapfreely ) on GitHub, respectively; but both seem empty.

Wondering the same. It seems like the answer is no but maybe it's because I'm not logged in?