This is neat--what's the typical refresh rate/update speed for the image service?

Is the API is basically "Push PNG to server, PNG displays on whiteboard?"

No, TRMNL generate the images themselves. The device requests the image every so often (with an interval configurable by the server). They've (understandably) built the firmware to be pretty specific to their service, though you can build your own server for it as well (it's just a JSON response with the image to fetch).

The API is:

You write HTML templates (that are stored on their servers) then can post data to them (or have them poll an endpoint to get the data) to "merge" into the template.

Think:

    <h1>Hi {{ username }}</h1>
    <div>You have {{ openPRCount }} open PRs</div>

    {"username": "joshstrange", "openPRCount": 10}

They provide a "UI Library" (Design System) [0] to give you building blocks of UI to use if you want.

It made it really easy to get started. They limit to 1 screen update every 15min (that is the fastest you can have the screen update). But you can press the restart button on the back of the device to force it to update, you might need to force your plugin to re-generate the image before you do that.

If you want to flash your own firmware you can lower that (at the expense of battery life) value or even have it call out to your own server that can do whatever you want.

[0] https://usetrmnl.com/framework