Image may be NSFW.
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An extra benefit of the Slack channel is that comments can be posted on a daily image, and reactions can be given in the form of emoticons. The Slack integration was written as a Python script, which runs daily at 8:45. The script is available at https://git.astron.nl/dailyimagebot.
After implementing this script, I learned that there is also an RSS-version of the daily image, as well as an RSS plugin for Slack. So that would probably have been an easier solution (which is also less fun to develop).