background-image
https://gyazo.com/24a8a86388d7593cdb3842ebd850681d
https://gyazo.com/e3f91c881ba931e6581c9381670a6203
https://placeholder.pics/svg/200x200/DEDEDE-FFFFFF/DEDEDE/200x200#.svg
https://placehold.jp/200x200.png?text=200x200
ランダム画像(画像にもよるが、主張が強すぎるようだ)
https://picsum.photos/200?grayscale#.png
https://source.unsplash.com/random/200x200?gray#.png