That error means Opera's desktop browser is blocking the image because it's trying to load an unencrypted http:// image link inside an https:// page. The mobile app is likely ignoring this, but PC browsers automatically block mixed content for security.

Since the image URL originates from the sender side, you can fix it on PC by going to Opera Settings > Privacy & security > Site Settings > Additional content settings > Insecure content, and adding https://web.flow.opera.com to the "Allowed to show insecure content" list. Reload My Flow, and the previews should render immediately.