You could make a custom anonymous page of login
Perhaps the Background Image Widget from the Native Mobile Resources module can be of help to you
I’d be very interested to know if you worked this out as I have the same issue. I’m using the Background Image widget which works well but I can’t get it to cover the whole screen in the same way it looks like you’re trying to do. I’m suspecting I need to make use of “react native” and set the height of the container but if that is the solution then it appears I have a lot more reading to do to work out how to use that (and I’m not even sure that is the solution or not)
export const loginBackground = {
container: {
height: "100%"
},
};
Add loginBackground to the background image widget class on the page.