the size of (1080 * 1920) broundground image can't show.

My window’s size is 1080 * 1920.
The broundground image is the same size.
But i can’t show the broundground.
If i change the window’s size to 1080 * 1720 ,the broundground will be show.
How to show the broundground when the window’s size is 1080 * 1920 ?