Its just a media integration texture. it will show whatever album is playing on spotify or a thumbnail from whatever youtube video is playing. theres an option to disable it in the settings.

"how to change the image showing on the TV"

You can't. Depends on the creator's knowledge about WPE. It is possible to add an option to replace an image in the wallpaper. It just needs to be implemented, nothing you can do about it without getting your hands on the source files. ^^

Compared to the surrounding environment, the image showing on TV is too abrupt.

how to change the image showing on the TV