This is covered numerous times over the lifetime of this site, simply utilize the search feature (magnifying glass at the top right).
Quite simply, utilize the HTML Includes feature to inject HTML (may it be a table, div, or a simple image tag) to the header (Control Panel -> Display -> General -> HTML Includes (tab) -> Default Header):
<a href="https://example.com/"><img src="https://example.com/image.jpg" target="_blank" style="max-width:100%;"></a>
