Hey there,
Please see https://community.theme.co/kb/customization-best-practices/ first before starting with customization. The template you would need to override is _landmark-header.php located in x\framework\views\integrity. Basically, you would need to add your image banner under the line
<header class="x-header-landmark x-container max width">
that is under the is_category
WordPress conditional tag. Please seek help from a developer should you not feel comfortable making these changes yourself.
Thanks.