UPDATE: When the homepage has the enabled cover via the Studio->Pages->Homepage->Settings area, Artificer uses the modules/boonex/artificer/data/template/system/images/cover-default.svg and cover-default-dark.svg images only.
I replaced both of those files with a new SVG with identical name. Nothing changed on the site even after clearing cache
edit: checked a few hours later and now it showed up, apparently the server just needed some time. Thanks!