Image_picker pulls from assets instead of files

I think it’s probably because you’re not providing it any parameters. Try giving it like this:

.AboutUs__one-third-image {
  background: url('{{ section.settings.one-third-image | img_url: "master" }}');
    max-width: 920px;
    display: block;
}

That’ll output whatever the image’s intrinsic dimensions are. You can provide it a size though:

.AboutUs__one-third-image {
  background: url('{{ section.settings.one-third-image | img_url: "1920x" }}');
    max-width: 920px;
    display: block;
}

That would give you a 1920px wide version of whatever image you’re referrencing.