Hero image in custom header

I am trying to add a hero image to a custom header component. I want the hero image to change based off dark/light mode.

I saw in the color_definitions.scss there is a dark-light-choose() so I tried adding the images (*.png) to my workspace AND tried uploading it in the admin. I noticed that when I upload the images to admin they disappear after a short while in my local instance.

Are there any docs on how to upload hero images? Thanks!