You need to modify your header.php file inside your theme folder (modern by default) or the CSS file your theme is using and add the following
In case of HTML:
<style type="text/css">
url('{url of image}');
</style>
In case of CSS:
url('{url of image}');
It’s better to use a relative path when pointing to the url of the image, if not, you should use an absolute path to the image.