As admin, when you create a user, if your current theme is Dark* (DarkRed or DarkBlue or …), then the background image is not displayed.
The console logs says:
https://c6.valid.enalean.com/themes/FlamingParrot/images/account-confirmation/account-registration-feedback-background-darkred.png
Failed to load resource: the server responded with a status of 404 (Not Found)