Right click on the picture that あなた want as your 壁紙 then click on set as 壁紙 image. または if not, go on control Panel and change it from Display and Themes I think it's called.
u know stop joking it easy okay this is how あなた take a picture of what u want it to be and it says set at background and thats how uuuuuuuuuu get it i dont have to say it agian geash!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! and those who dont know s i post yall dum an byeeeeeee!!!!!!!!!!!!!! PEACE!!!!!!!!!!!!!!!!!!!!!!!!!