As @wxl has encountered on this post there are some issues with uploaded images. This is a known issue that our sysadmin @teward is working on. This is what Thomas has currently posted on our internal status page:
Ongoing Content-Security-Policy issues since Transition to S3 on Discourse
5 days ago
Discourse image embeds and uploads are uploaded to S3 and the site Content Security Policy seems to be causing some chaos there.
We’re in the process of identifying and fixing the issues as they come up.