Hmmm, don’t know what to say. I Installed Ubuntu 18.04 LTS twice with discourse, both times I could not get large images to show a preview. I done the exact same process except this time using Ubuntu 16.04 LTS and image previews worked fine for me. The assumption was Ubuntu 18.04 LTS was the problem, if its not I would be happy to be pointed at any troubleshooting tips to identify the root cause of the issue.