CSS后台属性无法在服务器上运行

I just uploaded my first site that I created to a server, but not everything is working. I have a couple of pictures which are inserted using the CSS "background : url() " property but for some reason the pictures aren't showing up. I've tried putting the pictures in different folders and changing paths. The pictures are currently in the main directory so this is how my code looks like: I just uploaded my first site that I created to