First, that isn’t a valid baseURL unless you are creating a server-less site, which is rare:
https://gohugo.io/getting-started/configuration/#baseurl
Second, the assets directory is for files passed through an asset pipeline; you’re not doing that.
https://gohugo.io/getting-started/directory-structure/#directories
Assuming your baseURL will not contain a subdirectory, place your image in the static directory in the root of your project
static/images/bg.svg
Finally, depending on the SVG, you may have to add width/height to your CSS rule.