Millions of enterprise users visit Azure AD and Microsoft 365 sign-in screens everyday. In order to improve the sign-in experience, Microsoft today announced the new default background image on the sign-in screens. Microsoft mentioned that this new default background is one percent the size of the previous image, allowing the page to load faster on slower networks. Microsoft is planning to roll out this change in April. If your organization has customized the sign-in page, then this new default background change will not affect the visual look of the sign-in page.
This is solely a visual user interface (UI) change with no changes to functionality.