Today I'm working towards this by starting a Snap.as CLI project (https://github.com/snapas/snapas-cli) that'll work on its own, but I'll also include in the Write.as CLI. The latter will upload to Snap.as today, and when https://writeas.org is released, you'll have the option to upload photos to your own instance instead.
As an aside to that: like Write.as, Snap.as will have an open API so you can build on top of it. I'm aiming to have the API design solidified and documented by the end of this month. And I'd like to build it so the endpoints can blend with the Write.as API, so any apps built today will continue working with your self-hosted blog.