Have a look at the Style Your Mendix Native App documentation
https://docs.mendix.com/howto/mobile/how-to-use-native-styling and search for related forum posts like
To use an image as a background, you should use an mendix image widget. You can do it in 2 ways:
export const customImage = {
container: {
position: 'absolute',
},
};
export const customContainer = {
container: {
position: 'absolute',
},
};
Resources: