Where is the background image set in GameScreen of UnrealMatch3


I am a beginner of learning Unreal Engine, and I have a question during learning the sample “UnrealMatch3” as below.

It can be known that Match3Background is used by GameScreen through “Reference Viewer”. But I cannot find where the background image is set from the “Hierarchy” ,

屏幕快照 2017-05-18 09.24.14.png

Who can tell me how and where to set the background image of GameScreen?

Tiger Y