I am genuinely curious how you would prevent your site from being blocked like this? Sure in this case it was a demo, but what if it was an actual image hosting service that required login? What's to stop a bad actor from creating an account and uploading malicious content. Maybe you even have some filter that does image recognition on the images and tries to detect if it is phishing. But unless your filter is able to catch the exact same content that google's malware detector uses, there's still a chance that you'll miss something that google finds, and starts blocking your site.