Hi! Can u pls try this? I used this on my older MacBook Air 2015 version, and it worked, but on my 2020 version, it's not working anymore. I have no idea why, and it was very cool:''(
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Have you tried the widget below?
http://tracesof.net/uebersicht-widgets/#RSSs_Ubersicht_Widget
works fine for me, you just need to add the RSS feeds you want in the config.json file (that's in the widget folder).
I'm not the expert here (my coding skills are limited) but from experience I believe you'll need some CSS handlers to manage positioning and refresh rate of your image. If you didn't have set a refresh rate, what you describe is normal, Ubersicht will not dynamically pull the new image when updates as it will not refresh the widget.
See http://tracesof.net/uebersicht-widgets/#profile_widget
the coding for this widget is very similar with yours and it may give you some hints regarding the CSS you require to add.