Can you click on the Advanced settings buttons in the browser plugin’s option dialog and change the wait option to “Wait for connections idle”? Maybe that helps.
If you have access to the code generating those link to those two images: Can you temporarily try to remove the loading="lazy" attribute and see if that helps?
It seems like at least some network ranges of DigitalOcean (the hosting providers used to run the website snapshotting service) are blocked from accessing https://apidofa.fff.fr/ for reasons unknown. I’ve tried multiple machines from different data center locations and they all get a 403 (Forbidden) status code instead of the expected 200. As a result the embedded images are broken. Not sure what to do about this. I tried searching for a technical contact on the website, but didn’t find anything helpful.
In theory you could parse their API response, and replace the image urls with ones pointing to an image proxy endpoint on your own server. That way the images would be retrieved by your server (let’s hope that’s not running on DigitalOcean as well ) instead of the info-beamer servers. Note that you might run into legal issues regarding API licencing in that case.
If the HTML page is the only thing you want to display on your screens, you might also use the Fullscreen Browser package instead. Then it’s the Pi itself that’s retrieving those images.