Hello Lyndsay,
You have set the background image as cover. The percentage position will not work. With Background image you can hardly achieve a total responsiveness without compromises. I would like you to try this demo here: https://davidwalsh.name/demo/background-size.html which would explain how background image works.
In Cornerstone, we use the size cover
by default which would cover the whole area of the container. This is responsive enough which will always covers the whole area. The only thing to consider here is that if your image size is smaller, it will be stretch out and if you have a bigger image, it will get cut off.
Yes we can use the size 100% 100%
which is also responsive but your image will either be stretch out or get squeezed depending on the size of the container.
Yes we can also use the size 100% auto
but then as soon as you resize your browser or on smaller screen sizes, you will probably have white spaces around the image.
Or we can also use the size contain
but then this is not a good choice because you will see white spaces around your image.
The position could be top, left, right, center. and bottom. Percentage position may work too provided that you have set the size to auto.
https://www.w3schools.com/cssref/pr_background-position.asp
Hope this helps.