Graphic on password entry page for password protected pages

Site URL: https://www.welcomingthestranger.org/volunteerportal

I am helping a local non-profit with their website. There is a password protected page for volunteers, and another for students. The page that displays for password entry contains a graphic, which sizes properly on a phone screen, but extends off the page, top and bottom, when viewed on a computer screen. Have tested it with Chrome, Firefox and Edge, and it behaves the same on all of them.

Where would I find the settings for this "login" page, not the protected page? I would very much like to fix it as it looks stupid. My guess is that the problem occurs because a mobile browser is typically in portrait mode, while a computer is in landscape mode.

Any help would be most appreciated.

Cheers.

See Designing a Lock Screen.