Hello 
Never mind this, I just wrote this before I checked raw
Can you check the following site settings?
max image width default value: 690
max image height default value: 500
create thumbnails default value: ON
If you changed these settings before than it won’t affect the existing uploads so you have to do it manually.
If these settings are correct then you can instant check it with Rebuild HTML on the actual posts. Or try to upload a new image.
Rebuild HTML on Posts or Topics
If the thumbnail is generated correctly after these then you can bulk rebake the all posts.
cd /var/discourse/
./launcher enter app
rake posts:rebake
So I just checked your posts raw version. It looks like these images are added in composer with static HTML code. <img src"..." alt="...">. So these are not uploaded by using the default upload flow and it won’t generate thumbnail.
Did you try to just drag and drop image in composer or use the upload button?
However… Can you share more information about your setup? I see you are using Cloudflare which can cause issues depends on your setup. e.g. rocket loader usually break things… The uploads goes to local or S3? etc…
Is there any error in browser console when you upload an image?