Missing images in alerts notifications sent to Discord

I’m using self-hosted Grafana on Ubuntu 18.04. After I upgraded to 8.4.0 and switched to unified alerting, there are no images attached to discord notifications. Now I’m on Grafana 8.4.1 (grafana-image-renderer 3.4.1). The alerts were migrated from legacy ones. The datasource is Influx. Any idea what might be wrong?

How it used to be:

Now it looks like this:

It seems that the support for including images was dropped: Add "include image" option into Grafana Alerting · Discussion #38030 · grafana/grafana · GitHub. Sad that it is not mentioned anywhere.

this is coming back in Grafana 9: