Header Image Not Showing Up

I’m using the default theme. I’ve gone into wp-content/themes/default/style.css and changed the part of the code where it says #header url(images/kubrick.jpg) or whatever and put in my own link https://uoregon.edu/~kvillalp/photos/banner.jpg and it’s not showing up. I had it working earlier but the image was too small so I had to resize it and now I can’t get it to work at all. I’m so confused as to what could’ve possibly changed from one point to the next, help please!