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

https://forum.mendix.com/link/questions/95838 this one

To use an image as a background, you should use an mendix image widget. You can do it in 2 ways:

• Add custom styles to the image widget where you use `position: absolute` to position the image behind the other content.
  Example:

  export const customImage = {
      container: {
          position: 'absolute',
      },
  };

  
   
• Place all content on the page (except for the image) inside a container, and add `position: absolute` to that container to position it in front of the image.
  Example:

  export const customContainer = {
      container: {
          position: 'absolute',
      },
  };

   

Resources:

• See How to use native styling if you don’t know how to add custom styles.
• See Native Style Cheatsheet for all available styling properties.