Learn how to create a full page background image with CSS.
Read on how to do it in this link: https://www.w3schools.com/howto/howto_css_full_page.asp
Full Page Image
Learn how to create a background image that covers the entire browser window. The following example shows a full-screen (and a half-screen) responsive background image:
Try full page background image: https://www.w3schools.com/howto/tryhow_css_fullpage_demo.htm
Try half page background image: https://www.w3schools.com/howto/tryhow_css_halfpage_demo.htm