Sorry to bump this, but did you get any chance to try?
Did some more tests. Adding #<timestamp> to the URL did not make it refresh, but adding ?<timestamp> did. However, the ? made the yr server reject the request. I assume that most servers would accept it, so maybe the ? should be used anyways.
I have a script fetching the image every 30 minutes, which I assume will work, but I think that solution is too complex. I noticed that the javascript code mentioned support of an image entity, so I will look into that next.
Sorry for replying to myself again, but I want to leave a solution if anyone else encounters this problem.
Using HomeAssistant’s generic camera as a proxy and including the image as a camera widget in HADashboard finally solved the problem!
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