Mobile

Hi Sir,
Thanks for your plugin,
Please,
I uploaded a background image size: 1920 x 1280

but when i open the site on the mobile devices then the image is not displayed on all the site, how can i add a css code that when i will open the site on the mobile devices then it will use the image that have have another size ?

Regards
A.J.

https://wordpress.org/plugins/wp-maintenance-mode/