hello!! to do both of those things you want to get an image url. you get the url by right clicking then copying image url.
to add background photo/gif:
body{ background-image: url("image url"); }
example: body{ background-image: url("https://lh3.googleusercontent.com/proxy/o-EidIMxqgaux7dWNq4gcrL6HCFyPCg7ETBEBYnL7qKTtb8L38_sdPxxGH7rFA0SuebAdCWJUaBRyw4WstJ9-oqGaSTelqi_PljJuN89hSGBhWqPcP8-_ziQtMILvGOuwtsJ1oBhJPXDDyrHjw"); }
to put an image in