Very nice set up. I would probably just code it to hook up to an Instagram content and scrape the images directly instead of going through Twitter though.
That's exactly how we went about it. We could do a full Instagram integration, but we never used IFTTT and it saved us some coding + kept the example codebase much simpler.
IFTTT does have downsides tho, very slow to push from Instagram to Twitter.
It doesn't run Linux - the devices themsleves run as a thin-clients, that connect to a server. The server converts webpages into data and streams them to the device.
The server is running vanilla Ubuntu with Visionect Packages.