Docs: keeping a camera's snapshot

docs/interface/dashboards.md

@@ -615,7 +615,14 @@ portal), or a node of a running flow reads its message. Once its flow wires
 `camera`, the device's page shows a **Camera** preview. A flow that reads
 `camera` keeps it sending all the time, about 300 GB a month through a cloud
 instance, so the Device node warns when a node in its flow reads a port that
-sends on demand. Have flows read `snapshot` or `person` instead.
+sends on demand. Have flows read `snapshot` or `person` instead. To keep the
+picture of whoever came by, a node reading `snapshot` can hand it on to a
+message of its own, which a media tile shows until the next one:
+
+```python
+def process(snapshot):
+    return {"visitor": snapshot}
+```
 
 The device is told which of its ports have demand on its live socket, and sends
 its frames there too, as described under [Live events](../code/api.md#live-events).