Is it possible to upload background images for specific Google Calendar events? It seems that Google is showing this on their latest Google Calendar Android app, but is it possible to do this just in Google’s standard web app (i.e., cal.google.com)?
In the new UI of Google Calendar, these background images are called Flairs (the name is similar to the one used for the icons in the old version, available thanks to the Event Flairs Lab and still working in the old UI).
As pointed in this page, you can obtain a preview of the images by replacing the [ID] with the name of the flair in the following url:
Note that the keywords depend on the language of your browser: Google Cal recognizes only those written in the language in use, not necessarily those in English.
Coming to your question, as far I know is possible customize Event Flairs icons for the old UI (e.g. see here), but not the background Flairs for the new UI.