Here you want to consider that you're trying to reference the image from the perspective of the CSS which will be different from the HTML. So the pathing is quite different from the HTML. You'll want to think about how to access the image from this CSS file. Only then will the image show up.
Wonderful work getting the images to show up in the HTML! :)
Awesome attempt at a relative image path here in your CSS! https://github.com/ceceliazhen/prj-rev-bwfs-dasmoto/blob/master/Craft/Resources/css/style.css#L14
Here you want to consider that you're trying to reference the image from the perspective of the CSS which will be different from the HTML. So the pathing is quite different from the HTML. You'll want to think about how to access the image from this CSS file. Only then will the image show up.
Wonderful work getting the images to show up in the HTML! :)